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Title: An Introduction to Machine Drawing and Design
Author: David Allan Low
Release Date: March 4, 2012 [EBook #39033]
Language: English
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AN INTRODUCTION
TO
MACHINE DRAWING
AND
DESIGN
BY
DAVID ALLAN LOW
(WHITWORTH SCHOLAR), M. INST. M.E.
HEAD MASTER OF THE PEOPLE'S PALACE TECHNICAL SCHOOLS, LONDON
AUTHOR OF 'A TEXT-BOOK ON PRACTICAL SOLID OR DESCRIPTIVE GEOMETRY'
'AN ELEMENTARY TEXT-BOOK OF APPLIED MECHANICS' ETC.
[Illustration]
_FOURTH EDITION_
LONDON
LONGMANS, GREEN, AND CO.
AND NEW YORK: 15 EAST 16th STREET
1890
PRINTED BY
SPOTTISWOODE AND CO., NEW-STREET SQUARE
LONDON
PREFACE.
It is now generally recognised that the old-fashioned method of teaching
machine drawing is very unsatisfactory. In teaching by this method an
undimensioned scale drawing, often of a very elaborate description, is
placed before the student, who is required to _copy_ it. Very often the
student succeeds in making a good copy of the drawing placed before him
without learning very much about the object represented by it, and this
state of matters is sometimes not much improved by the presence of the
teacher, who is often simply an art master, knowing nothing about
machine design. It is related of one school that a pupil, after making a
copy of a particular drawing, had a discussion with his teacher as to
whether the object represented was a sewing machine or an electrical
machine. Evidently the publisher of the drawing example in this case did
not adopt the precaution which a backward student used at an examination
in machine design: he put on a full title above his drawing, for the
information of his examiner.
Now, if machine drawing is to be of practical use to any one, he must be
able to understand the form and arrangement of the parts of a machine
from an inspection of suitable drawings of them without seeing the parts
themselves. Also he ought to be able to make suitable drawings of a
machine or parts of a machine from the machine or the parts themselves.
In producing this work the author has aimed at placing before young
engineers and others, who wish to acquire the skill and knowledge
necessary for making the simpler _working drawings_ such as are produced
in engineers' drawing offices, a number of good exercises in drawing,
sufficient for one session's work, and at the same time a corresponding
amount of information on the design of machine details generally.
The exercises set are of various kinds. In the first and simplest
certain views of some machine detail are given, generally drawn to a
small scale, which the student is asked to reproduce _to dimensions
marked on these views_, and he is expected to keep to these dimensions,
and not to measure anything from the given illustrations. In the second
kind of exercise the student is asked to reproduce certain views shown
_to dimensions given in words or in tabular form_. In the third kind of
exercise the student is required to make, in addition to certain views
shown to given dimensions, others which he can only draw correctly if he
thoroughly understands the design before him. In the fourth kind of
exercise the student is asked to make the necessary working drawings for
some part of a machine which has been previously described and
illustrated, _the dimensions to be calculated by rules given in the
text_.
The illustrations for this work are all new, and have been specially
prepared by the author from _working drawings_, and he believes that
they will be found to represent the best modern practice.
As exercises in drawing, those given in this book are not numbered
exactly in their order of difficulty, but unless on the recommendation
of a teacher, the student should take them up in the order given,
omitting the following:--26, 27, 28, 35, 40, 42, 43, 45, 49, 50, 54, 60,
61, as he comes to them, until he has been right through the book;
afterwards he should work out those which he omitted on first going over
the book.
In addition to the exercises given in this work the student should
practise making freehand sketches of machine details from actual
machines or good models of them. Upon these sketches he should put the
proper dimensions, got by direct measurement from the machine or model
by himself. These sketches should be made in a note-book kept for the
purpose, and no opportunity should be lost of inserting a sketch of any
design which may be new to the student, always putting on the dimensions
if possible. These sketches form excellent examples from which to make
working drawings. The student should also note any rules which he may
meet with for proportioning machines, taking care, however, in each case
to state the source of such information for his future guidance and
reference.
As machine drawing is simply the application of the principles of
descriptive geometry to the representation of machines, the student of
the former subject, if he is not already acquainted with the latter,
should commence to study it at once.
D. A. L.
GLASGOW: _March_ 1887.
_PREFACE TO THE THIRD EDITION._
To this edition another chapter has been added, containing a number of
miscellaneous exercises, which it is hoped will add to the usefulness of
the work as a text-book in science classes. The latest examination paper
in machine drawing by the Science and Art Department has also been added
to the Appendix.
D. A. L.
LONDON: _August_ 1888.
CONTENTS.
PAGE
I. INTRODUCTION 1
II. RIVETED JOINTS 6
III. SCREWS, BOLTS, AND NUTS 14
IV. KEYS 22
V. SHAFTING 24
VI. SHAFT COUPLINGS 25
VII. BEARINGS FOR SHAFTS 30
VIII. PULLEYS 36
IX. TOOTHED WHEELS 39
X. CRANKS AND CRANKED SHAFTS 43
XI. ECCENTRICS 47
XII. CONNECTING RODS 49
XIII. CROSS-HEADS 56
XIV. PISTONS 57
XV. STUFFING-BOXES 63
XVI. VALVES 68
XVII. MATERIALS USED IN MACHINE CONSTRUCTION 76
XVIII. MISCELLANEOUS EXERCISES 81
APPENDIX A 99
APPENDIX B 102
INDEX 113
AN INTRODUCTION
TO
MACHINE DRAWING AND DESIGN.
I. INTRODUCTION.
_Drawing Instruments._--For working the exercises in this book the
student should be provided with the following:--A well-seasoned yellow
pine _drawing-board_, 24 inches long, 17 inches wide, and 3/8 inch or
1/2 inch thick, provided with cross-bars on the back to give it strength
and to prevent warping. A =T= _square_, with a blade 24 inches long
attached permanently to the stock, _but not sunk into it_. One 45° and
one 60° _set square_. The short edges of the former may be about 6
inches and the short edge of the latter about 5 inches long. A _pair of
compasses_ with pen and pencil attachments, and having legs from 5
inches to 6 inches long. A _pair of dividers_, with screw adjustment if
possible. A _pair of small steel spring pencil bows_ for drawing small
circles, and a _pair of small steel spring pen bows_ for inking in the
same. A _drawing pen_ for inking in straight lines. All compasses should
have _round points_, and if possible _needle_ points. A piece of
india-rubber will also be required, besides two pencils, one marked H or
HH and one marked HB or F; the latter to be used for lining in a drawing
which is not to be inked in, or for freehand work.
Pencils for mechanical drawing should be sharpened with a _chisel
point_, and those for freehand work with a _round point_. _Do not wet
the pencil_, as the lines afterwards made with it are very difficult to
rub out.
Drawing-paper for working drawings may be secured to the board by
_drawing-pins_, but the paper for finished drawings or drawings upon
which there is to be a large amount of colouring should be _stretched_
upon the board.
The student should get the best instruments he can afford to buy, and he
should rather have a few good instruments than a large box of inferior
ones.
_Drawing-paper._--The names and sizes of the sheets of drawing paper are
given in the following table:--
Inches
Demy 20 × 15
Medium 22 × 17
Royal 24 × 19
Imperial 30 × 22
Atlas 34 × 26
Double Elephant 40 × 27
Antiquarian 52 × 31
The above sizes must not be taken as exact. In practice they will be
found to vary in some cases as much as an inch.
Cartridge-paper is made in sheets of various sizes, and also in rolls.
Hand-made paper is the best, but it is expensive. Good cartridge-paper
is quite suitable for ordinary drawings.
_Centre Lines._--Drawings of most parts of machines will be found to
be symmetrical about certain lines called _centre lines_. These lines
should be drawn first with great care. On a pencil drawing centre
lines should be thin continuous lines; in this book they are shown
thus -- - -- - --.
After drawing the centre line of any part the dimensions of that part
must be marked off from the centre line, so as to insure that it really
is the centre line of that part: thus in making a drawing of a rivet,
such as is shown at (_a_) fig. 1, after drawing the centre line, half
the diameter of the rivet would be marked off on each side of that line,
in order to determine the lines for the sides of the rivet.
_Inking._--For inking in drawings the best Indian ink should be used,
and not common writing ink. Common ink does not dry quick enough, and
rapidly corrodes the drawing pens. The pen should be filled by means of
a brush or a narrow strip of paper, and not by dipping the pen into the
ink.
In cases where there are straight lines and arcs of circles touching one
another _ink in the arcs first_, then the straight lines; in this way it
is easier to hide the joints.
_Colouring._--Camel's-hair or sable brushes should be used; the latter
are the best, but are much more expensive than the former. The colour
should be rubbed down in a dish, and the tint should be light. The
mistake which a beginner invariably makes is in having the colour of too
dark a tint.
First go over the part to be coloured with the brush and _clean_ water
for the purpose of damping it. Next dry with clean blotting-paper to
take off any superfluous water. Then take another brush with the colour,
and beginning at the top, work from left to right and downwards. If it
is necessary to recolour any part let the first coating dry before
beginning.
Engineers have adopted certain colours to represent particular
materials; these are given in the following table:--
_Table showing Colours used to represent Different Materials._
MATERIAL COLOUR
Cast iron Payne's grey or neutral tint.
Wrought iron Prussian blue.
Steel Purple (mixture of Prussian blue and crimson lake).
Brass Gamboge with a little sienna or a very little red
added.
Copper A mixture of crimson lake and gamboge, the former
colour predominating.
Lead Light Indian ink with a very little indigo added.
Brickwork Crimson lake and burnt sienna.
Firebrick Yellow and Vandyke brown.
Greystones Light sepia or pale Indian ink, with a little
Prussian blue added.
Brown freestone Mixture of pale Indian ink, burnt sienna, and
carmine.
Soft woods For ground work, pale tint of sienna.
Hard woods For ground work, pale tint of sienna with a little
red added.
For graining woods use darker tint with a greater
proportion of red.
_Printing._--A good drawing should have its title printed, a plain style
of letter being used for this purpose, such as the following:--
[Illustration: ABCDEFGHIJKLMNOPQRST
UVWXYZ
1234567890]
[Illustration: ABCDEFGHIJKLMNOPQR
STUVWXYZ
1234567890]
The following letters look well _if they are well made_, but they are
much more difficult to draw.
[Illustration: ABCDEFGHIJKLMNOP
QRSTUVWXYZ
1234567890]
For remarks on a drawing the following style is most suitable:--
[Illustration: abcdefghijklmnopqrstuvwxyz]
All printing should be done by freehand.
_Border lines_ are seldom put on engineering drawings.
_Working Drawings._--A good working drawing should be prepared in the
following manner. It must first be carefully outlined in pencil and then
inked in. After this all parts cut by planes of section should be
coloured, the colours used indicating the materials of which the parts
are made. Parts which are round may also be lightly shaded with the
brush and colours to suit the materials. The centre lines are now inked
in with _red_ or _blue ink_. The red ink may be prepared by rubbing down
the cake of crimson lake, and the blue ink in like manner from the cake
of Prussian blue. Next come the _distance_ or _dimension_ lines, which
should be put in with _blue_ or _red ink_, depending on which colour was
used for the centre lines. Dimension lines and centre lines are best put
in of different colour. The arrow-heads at the ends of the dimension
lines are now put in with _black ink_, and so are the figures for the
dimensions. The arrow-heads and the figures should be made with a common
writing pen. The dimensions should be put on neatly. Many a good drawing
has its appearance spoiled through being slovenly dimensioned.
We may here point out the importance of putting the dimensions on a
working drawing. If the drawing is not dimensioned, the workman must get
his sizes from the drawing by applying his rule or a suitable scale. Now
this operation takes time, and is very liable to result in error. Time
is therefore saved, and the chance of error reduced, by marking the
sizes in figures.
In practice it is not usual to send original drawings from the drawing
office to the workshop, but copies only. The copies may be produced by
various 'processes,' or they may be tracings drawn by hand. Many
engineers do not ink in their original drawings, but leave them in
pencil; especially is this the case if the drawings are not likely to be
much used.
_Scales._--The best scales are made of ivory, and are twelve inches
long. Boxwood scales are much cheaper, although not so durable as those
made of ivory. If the student does not care to go to the expense of
ivory or boxwood scales, he can get paper ones very cheap, which will be
quite sufficient for his purpose. The divisions of the scale should be
marked down to its edge, so that measurements may be made by applying
the scale directly to the drawing. For working such exercises as are in
this book the student should be provided with the following scales:--
A scale of 1, or 12 inches to a foot.
" 1/2 " 6 "
" 1/3 " 4 "
" 1/4 " 3 "
" 1/6 " 2 "
A scale of 1 is spoken of as 'full size,' and a scale of 1/2 as 'half
size.'
Engineers in this country state dimensions of machines in feet, inches,
and fractions of an inch, the latter being the 1/2, 1/4, 1/8, 1/16, &c.
In making calculations it is generally more convenient to use decimal
fractions, and then substitute for the results the equivalent fractions
in eighths, sixteenths, &c. The following table will be found useful for
this purpose:--
_Decimal Equivalents of Fractions of an Inch._
+----------+--------------------+
| Fraction | Decimal Equivalent |
+----------+--------------------+
| 1/32 | .03125 |
| 1/16 | .0625 |
| 3/32 | .09375 |
| 1/8 | .125 |
| 5/32 | .15625 |
| 3/16 | .1875 |
| 7/32 | .21875 |
| 1/4 | .25 |
| 9/32 | .28125 |
| 5/16 | .3125 |
| 11/32 | .34375 |
| 3/8 | .375 |
| 13/32 | .40625 |
| 7/16 | .4375 |
| 15/32 | .46875 |
| 1/2 | .5 |
| 17/32 | .53125 |
| 9/16 | .5625 |
| 19/32 | .59375 |
| 5/8 | .625 |
| 21/32 | .65625 |
| 11/16 | .6875 |
| 23/32 | .71875 |
| 3/4 | .75 |
| 25/32 | .78125 |
| 13/16 | .8125 |
| 27/32 | .84375 |
| 7/8 | .875 |
| 29/32 | .90625 |
| 15/16 | .9375 |
| 31/32 | .96875 |
| 1 | 1.0 |
+----------+--------------------+
Engineers use a single accent (') to denote _feet_, and a double
accent (") to denote _inches_. Thus 2' 9" reads two feet nine inches.
II. RIVETED JOINTS.
Two plates or pieces to be riveted together have holes punched or
drilled in them in such a manner that one may be made to overlap the
other so that the holes in the one may be opposite the holes in the
other. The rivets, which are round bars of iron, or steel, or other
metal, are heated to redness and inserted in the holes; the head already
formed on the rivet, and called the tail, is then held up, and the point
is hammered or pressed so as to form another head. This process of
forming the second head on the rivet is known as riveting, and may be
done by hand-hammering or by a machine.
_Forms of Rivet Heads._--In fig. 1 are shown four different forms of
rivet heads: (_a_) is a _snap head_, (_b_) a _conical head_ (_c_) a
_pan head_, and (_d_) _a countersunk head_.
_Proportions of Rivet Heads._--The diameter of the snap head is about
1.7 times the diameter of the rivet, and its height about .6 of the
diameter of the rivet. The conical head has a diameter twice and a
height three quarters of the rivet diameter. The greatest diameter of
the pan head is about 1.6, and its height .7 of the rivet diameter. The
greatest diameter of the countersunk head may be one and a half, and its
depth a half of the diameter of the rivet.
[Illustration: FIG. 1.]
In fig. 1 at (_a_) and (_b_) are shown geometrical constructions devised
by the author for drawing the snap and conical head for any size of
rivet, the proportions being nearly the same as those given above.
_Geometrical Construction for Proportioning Snap Heads._--With centre A,
and radius equal to half diameter of rivet, describe a circle cutting
the centre line of the rivet at B and C. With centre B and radius BC
describe the arc CD. Make BE equal to AD. With centre E and radius ED
describe the arc DFH.
_Construction for Conical Head._--With centre K, and radius equal to
diameter of rivet, describe the semicircle LMN, cutting the side of the
rivet at M. With centre M and radius MN describe the arc NP to cut the
centre line of rivet at P. Join PL and PN.
When a number of rivets of the same diameter have to be shown on the
same drawing the above constructions need only be performed on one
rivet. After the point E has been discovered the distance AE may be
measured off on all the other rivets, and the arcs corresponding to
DFH drawn with radii equal to ED. In like manner the height KP of the
conical head may be marked off on all rivets of the same diameter with
conical heads.
_Caulking._--In order to make riveted joints steam- or water-tight the
edges of the plates and the edges of the heads of the rivets are burred
down by a blunt chisel or caulking tool as shown at Q and R.
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
EXERCISE 1: _Forms of Rivets._--Draw, full size, the rivets and
rivet heads shown in fig. 1. The diameter of the rivet in each
case to be 1-1/8 inches, and the thickness of the plates 7/8 inch.
EXERCISE 2: _Single Riveted Lap Joint._--Draw, full size, the
plan and sectional elevation of the _single riveted lap joint_
shown in fig. 2.
_Table showing the Proportions of Single Riveted Lap Joints for various
Thicknesses of Plates._ (_Plates and Rivets Wrought Iron._)
+--------------+-------------+----------+--------------+
| Thickness of | Diameter of | Pitch of | Width of lap |
| plates | rivets | rivets | |
+--------------+-------------+----------+--------------+
| 1/4 | 9/16 | 1-5/8 | 1-3/4 |
| 5/16 | 5/8 | 1-3/4 | 2 |
| 3/8 | 11/16 | 1-7/8 | 2-1/4 |
| 7/16 | 3/4 | 2 | 2-1/2 |
| 1/2 | 13/16 | 2-1/8 | 2-3/4 |
| 9/16 | 7/8 | 2-1/4 | 2-7/8 |
| 5/8 | 15/16 | 2-5/16 | 3 |
| 11/16 | 1 | 2-3/8 | 3-1/8 |
| 3/4 | 1-1/16 | 2-1/2 | 3-1/4 |
+------------------------------------------------------+
All the dimensions are in inches.
[Illustration: FIG. 4.]
EXERCISE 3.--Draw, half size, a plan and section of a single
riveted lap joint for plates 3/4" thick to the dimensions given in
the above table.
EXERCISE 4: _Double Riveted Lap Joint._--Draw, full size, the two
views of the _double riveted lap joint_ shown in fig. 3.
_Table showing the Proportions of Double Riveted Lap Joints for various
Thicknesses of Plates._ (_Plates and Rivets Wrought Iron._)
+-----------+-------------+----------+------------------+----------+
| Thickness | Diameter of | Pitch of | Distance between | Width of |
| of plates | rivets | rivets | rows of rivets | lap |
+-----------+-------------+----------+------------------+----------+
| 3/8 | 11/16 | 2-1/2 | 1-1/8 | 3-1/2 |
| 7/16 | 3/4 | 2-5/8 | 1-1/4 | 3-3/4 |
| 1/2 | 13/16 | 2-3/4 | 1-3/8 | 4 |
| 9/16 | 7/8 | 2-7/8 | 1-7/16 | 4-1/4 |
| 5/8 | 15/16 | 3 | 1-9/16 | 4-1/2 |
| 11/16 | 1 | 3-1/8 | 1-3/4 | 4-3/4 |
| 3/4 | 1-1/16 | 3-1/4 | 1-7/8 | 5 |
| 13/16 | 1-1/16 | 3-3/8 | 1-7/8 | 5 |
| 7/8 | 1-1/8 | 3-1/2 | 1-15/16 | 5-1/4 |
| 15/16 | 1-1/8 | 3-5/8 | 1-15/16 | 5-1/4 |
| 1 | 1-3/16 | 3-3/4 | 2 | 5-1/2 |
+-----------+-------------+----------+------------------+----------+
[Illustration: FIG. 5.]
EXERCISE 5.--Draw, half size, a plan and section of a double
riveted lap joint for plates 7/8 inch thick to the dimensions
given in the above table.
EXERCISE 6: _Single Riveted Butt Joints._--In fig. 4 are shown
_single riveted butt joints_. One of the sectional views shows a
butt joint with one _cover plate_ or _butt strap_; the other
sectional view shows the same joint with two cover plates; the
third view is a plan of both arrangements. Draw all these views
full size.
EXERCISE 7.--Fig. 5 shows a plan and sectional elevation of the
connection of three plates together, which are in the same plane,
by means of single riveted butt joints and single cover plates.
The butt straps where they overlap are forged so as to fit one
another as shown, and thus form a close joint. Draw these views to
the scale of 6 inches to a foot.
The plates are 1/2 inch thick and the butt straps 9/16 inch thick.
All other dimensions must be deduced from the table for single
riveted lap joints.
EXERCISE 8.--The connection of three plates by single riveted lap
joints is shown in fig. 6. To make the joint close one plate has a
portion of its edge thinned out, and the plate above it is set up
at this part so as to lie close to the former.
Draw the three views shown in fig. 6 to the same scale as the last
exercise.
The plates are 7/16 inch thick. All other dimensions to be
obtained from table for single riveted lap joints.
EXERCISE 9: _Corner of Wrought-iron Tank._--This exercise is to
illustrate the connection of plates which are at right angles to
one another by means of _angle irons_. Fig. 7 is a plan and
elevation of the corner of a wrought-iron tank. The sides of the
tank are riveted to a vertical angle iron, the cross section of
which is clearly shown in the plan. Another angle iron of the same
dimensions is used in the same way to connect the sides with the
bottom. The sides do not come quite up to the corner of the
vertical angle iron, excepting at the bottom where the horizontal
angle iron comes in. At this point the vertical plates meet one
another, and the edge formed is rounded over to fit the interior
of the bend of the horizontal angle iron so as to make the joint
tight. Draw half size.
The dimensions are as follows: angle irons 2-1/2 inches × 2-1/2
inches × 3/8 inch; plates 3/8 inch thick; rivets 11/16 inch
diameter and 2 inches pitch.
EXERCISE 10: _Gusset Stay._--In order that the flat ends of a
steam boiler may not be bulged out by the pressure of the steam
they are strengthened by means of stays. One form of boiler stay,
called a 'gusset stay,' is shown in fig. 8. This stay consists of
a strip of wrought-iron plate which passes in a diagonal direction
from the flat end of the boiler to the cylindrical shell. One end
of this plate is placed between and riveted to two angle irons
which are riveted to the shell of the boiler. A similar
arrangement connects the other end of the stay plate to the flat
end of the boiler. In this example the stay or gusset plate is 3/4
of an inch thick; the angle irons are 4 inches broad and 1/2 inch
thick. The rivets are 1 inch in diameter. The same figure also
illustrates the most common method of connecting the ends of a
boiler to the shell. The end plates are _flanged_ or bent over at
right angles and riveted to the shell as shown. The radius of the
inside curve at the angle of the flange is 1-1/4 inches. Draw this
example to a scale of 3 inches to 1 foot.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
III. SCREWS, BOLTS, AND NUTS.
_Screw Threads._--The various forms of screw threads used in machine
construction are shown in fig. 9. The _Whitworth_ =V= thread is shown at
(_a_). This is the standard form of triangular thread used in this
country. The angle between the sides of the =V= is 55°, and one-sixth of
the total depth is rounded off both at the top and bottom. At (_b_) is
shown the _Sellers_ =V= thread, which is the standard triangular thread
used by engineers in America. In this form of thread the angle between
the sides of the =V= is 60°, and one-eighth of the total depth is cut
square off at the top and bottom. The _Square_ thread is shown at (_c_).
This form is principally used for transmitting motion.
[Illustration: FIG. 9.]
Comparing the triangular and square threads, the former is the stronger
of the two; but owing to the normal pressure on the =V= thread being
inclined to the axis of the screw, that pressure must be greater than
the pressure which is being transmitted by the screw; and therefore,
seeing that the normal pressure on the square thread is parallel, and
therefore equal to the pressure transmitted in the direction of the axis
of the screw, the friction of the =V= thread must be greater than the
friction of the square thread. In the case of the triangular thread
there is also a tendency of the pressure to burst the nut. The
_Buttress_ thread shown at (_e_) is designed to combine the advantages
of the =V= and square threads, but it only has these advantages when the
pressure is transmitted in one direction; if the direction of the
pressure be reversed, the friction and bursting action on the nut are
even greater than with the =V= thread, because of the greater
inclination of the slant side of the buttress thread. The angles of the
square thread are frequently rounded to a greater or less extent to
render them less easily damaged. If this rounding is carried to excess
we get the _Knuckle_ thread shown at (_d_). The rounding of the angles
increases both the strength and the friction.
EXERCISE 11: _Forms of Screw Threads._--Draw to a scale of three
times full size the sections of screw threads as shown in fig. 9.
The pitch for the Whitworth, Sellers, and buttress threads to be
3/8 inch, and the pitch of the square and knuckle threads to be 1/2
inch.
_Dimensions of Whitworth Screws._
+-----------------------------------+
| Diameter | Number | Diameter |
| of screw | of threads | at bottom |
| | per inch | of thread |
+----------+------------+-----------+
| 1/8 | 40 | .093 |
| 3/16 | 24 | .134 |
| 1/4 | 20 | .186 |
| 5/16 | 18 | .241 |
| 3/8 | 16 | .295 |
| 7/16 | 14 | .346 |
| 1/2 | 12 | .393 |
| 5/8 | 11 | .508 |
| 3/4 | 10 | .622 |
| 7/8 | 9 | .733 |
| 1 | 8 | .840 |
| 1-1/8 | 7 | .942 |
| 1-1/4 | 7 | 1.067 |
| 1-3/8 | 6 | 1.162 |
| 1-1/2 | 6 | 1.286 |
| 1-5/8 | 5 | 1.369 |
| 1-3/4 | 5 | 1.494 |
| 1-7/8 | 4-1/2 | 1.590 |
| 2 | 4-1/2 | 1.715 |
| 2-1/4 | 4 | 1.930 |
| 2-1/2 | 4 | 2.180 |
| 2-3/4 | 3-1/2 | 2.384 |
| 3 | 3-1/2 | 2.634 |
| 3-1/4 | 3-1/4 | 2.856 |
| 3-1/2 | 3-1/4 | 3.106 |
| 3-3/4 | 3 | 3.323 |
| 4 | 3 | 3.573 |
| 4-1/4 | 2-7/8 | 3.805 |
| 4-1/2 | 2-7/8 | 4.055 |
| 4-3/4 | 2-3/4 | 4.284 |
| 5 | 2-3/4 | 4.534 |
| 5-1/4 | 2-5/8 | 4.762 |
| 5-1/2 | 2-5/8 | 5.012 |
| 5-3/4 | 2-1/2 | 5.238 |
| 6 | 2-1/2 | 5.488 |
+-----------------------------------+
_Gas Threads_[1] (_Whitworth Standard_).
[1] Used for wrought-iron and brass tubes.
+-------------------------------------------------------------+
| Diameter of Screw | 1/8 | 1/4 | 3/8 | 1/2 | 5/8 | 3/4 | 1 |
+-------------------+-----+-----+-----+-----+-----+-----+------
| Number of threads | | | | | | | |
| per inch | 28 | 19 | 19 | 14 | 14 | 14 | 11 |
+-------------------------------------------------------------+
+-------------------------------------------------+
| Diameter of Screw | 1-1/4 | 1-1/2 | 1-3/4 | 2 |
+-------------------+-------+-------+-------+-----+
| Number of threads | | | | |
| per inch | 11 | 11 | 11 | 11 |
+-------------------------------------------------+
_Representation of Screws._--The correct method of representing screw
threads involves considerable trouble, and is seldom adopted by
engineers for working drawings. For an explanation of the method see the
author's Text-book on Practical Solid Geometry, Part II., problem 134. A
method very often adopted on working drawings is shown in fig. 15; here
the thin lines represent the points, and the thick lines the roots of
the threads. At fig. 16 is shown a more complete method. The simplest
method is illustrated by figs. 10, 11, 13, and 14.
Here dotted lines are drawn parallel to the axis of the screw as far as
it extends, and at a distance from one another equal to the diameter of
the screw at the bottom of the thread.
[Illustration: FIG. 10.]
[Illustration: FIG. 11.]
_Forms of Nuts._--The most common form of nut is the hexagonal shown in
figs. 10, 13, 14, 15, and 16; next to this comes the square nut shown in
fig. 11. The method of drawing these nuts will be understood by
reference to the figures; the small circles indicate the centres, and
the inclined lines passing through them the radii of the curves which
represent the chamfered or bevelled edge of the nut. In all the figures
but the first the chamfer is just sufficient to touch the middle points
of the sides, and in these cases the drawing of the nut is simpler.
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
[Illustration: FIG. 14.]
_Forms of Bolts._--At (_a_), fig. 12, is shown a bolt with a square head
and a square neck. If this form of bolt is passed through a square hole
the square neck prevents the bolt from turning when the nut is being
screwed up. Instead of a square neck a snug may be used for the same
purpose, as shown on the cup-headed bolt at (_b_). The snug fits into a
short groove cut in the side of the hole through which the bolt passes.
At (_a_) the diagonal lines are used to distinguish the flat side of the
neck from the round part of the bolt above it. At (_c_) is shown a
tee-headed bolt, and at (_d_) an eye-bolt. Fig. 13 represents a hook
bolt. A bolt with a countersunk head is shown in fig. 11. If the
countersunk head be lengthened so as to take up the whole of the
unscrewed part of the bolt, we get the taper bolt shown in fig. 14,
which is often used in the couplings of the screw shafts of steamships.
The taper bolt has the advantage of having no projecting head, and it
may also be made a tight fit in the hole with less trouble than a
parallel bolt. Bolts may also have hexagonal heads.
[Illustration: FIG. 15]
[Illustration: FIG. 16]
_Studs_, or _stud bolts_, are shown in figs. 15 and 16; that in fig. 15
is a _plain stud_, while that in fig. 16 has an intermediate collar
forged upon it, and is therefore called a _collared stud_.
_Proportions of Nuts and Bolt-heads._--In the hexagonal nut the diameter
D across the flats is 1-1/2_d_ + 1/8, where _d_ is the diameter of the
bolt. The same rule gives the width of a square nut across the flats. A
rule very commonly used in making drawings of hexagonal nuts is to make
the diameter D, across the angles equal to 2_d_. H, the height of the
nut, is equal to the diameter of the bolt. In square and hexagonal
headed bolts the height of the head varies from _d_ to 2/3_d_; the other
dimensions are the same as for the corresponding nuts.
_Washers_ are flat, circular, wrought-iron plates, having holes in their
centres of the same diameter as the bolts on which they are used. The
object of the washer is to give a smooth bearing surface for the nut to
turn upon, and it is used when the surfaces of the pieces to be
connected are rough, or when the bolt passes through a hole larger than
itself, as shown in fig. 10. The diameter of the washer is a little more
than the diameter of the nut across the angles, and its thickness about
1/8 of the diameter of the bolt.
EXERCISE 12.--Draw, full size, the views shown in fig. 10 of an
hexagonal nut and washer for a bolt 1-1/4 inches in diameter. The
bolt passes through a hole 1-3/4 × 1-1/4. All the dimensions are
to be calculated from the rules which have just been given.
EXERCISE 13.--Draw, full size, the plan and elevation of the
square nut and bolt with countersunk head shown in fig. 11, to the
dimensions given.
EXERCISE 14.--Draw, full size, the elevation of the hook bolt with
hexagonal nut shown in fig. 13 to the dimensions given, and show
also a plan.
EXERCISE 15.--Draw, to a scale of 4 inches to a foot, the conical
bolt for a marine shaft coupling shown in fig. 14. All the parts
are of wrought iron.
EXERCISE 16.--Fig. 15 is a section of the mouth of a small
steam-engine cylinder, showing how the cover is attached; draw
this full size.
EXERCISE 17.--Fig. 16 shows the central portion of the
india-rubber disc valve which is described on page 68. A is the
central boss of the grating, into which is screwed the stud B,
upon which is forged the collar C. The upper part of the stud is
screwed, and carries the guard D and an hexagonal nut E. F is the
india-rubber. The grating and guard are of brass. The stud and nut
are of wrought iron. Draw full size the view shown.
_Lock Nuts._--In order that a nut may turn freely upon a bolt, there is
always a very small clearance space between the threads of the nut and
those of the bolt. This clearance is shown exaggerated at (_a_), fig.
17, where A is a portion of a bolt within a nut B. Suppose that the bolt
is stretched by a force W. When the nut B is screwed up, the upper
surfaces of the projecting threads of the nut will press on the under
surfaces of the threads of the bolt with a force P equal and opposite
to W, as shown at (_b_), fig. 17. When in this condition the nut has no
tendency to slacken back, because of the friction due to the pressure on
the nut. Now suppose that the tension W on the bolt is momentarily
diminished, then the friction which opposes the turning of the nut may
be so much diminished that a vibration may cause it to slacken back
through a small angle. If this is repeated a great many times the nut
may slacken back so far as to become useless.
[Illustration: FIG. 17.]
[Illustration: FIG. 18.]
A very common arrangement for locking a nut is shown at (_a_), fig. 18.
C is an ordinary nut, and B one having half the thickness of C. B is
first screwed up tight so as to act on the bolt, as shown at (_b_), fig.
17. C is then screwed on top of B. When C is almost as tight as it can
be made, it is held by one spanner, while B is turned back through a
small angle with another. The action of the nuts upon the bolt and upon
one another is now as shown at (_b_), fig. 18. It will be seen that the
nuts are wedged tight on to the bolt, and that this action is
independent of the tension W in the bolt. The nuts will, therefore,
remain tight after the tension in the bolt is removed.
It is evident that if the nuts are screwed up in the manner explained,
the outer nut C will carry the whole load on the bolt; hence C should be
the thicker of the two nuts. In practice, the thin nut, called the lock
nut, is often placed on the outside, for the reason that ordinary
spanners are too thick to act on the thin nut when placed under the
other.
Another very common arrangement for locking a nut is shown in fig. 19. A
is the bolt and B the nut, the lower part of which is turned circular. A
groove C is also turned on the nut at this part. The circular part of
the nut fits into a circular recess in one of the parts connected by the
bolt. Through this part passes a set screw D, the point of which can be
made to press on the nut at the bottom of the groove C. D is turned back
when the nut B is being moved, and when B is tightened up, the set screw
is screwed up so as to press hard on the bottom of the groove C. The nut
B is thus prevented from slackening back. The screw thread is turned off
the set screw at the point where it enters the groove on the nut.
[Illustration: FIG. 19]
The use of the groove for receiving the point of the set screw is this:
The point of the set screw indents the nut and raises a bur which would
interfere with the free turning of the nut in the recess if the bur was
not at the bottom of a groove. Additional security is obtained by
drilling a hole through the point of the bolt, and fitting it with a
split pin E.
Locking arrangements for nuts are exceedingly numerous, and many of them
are very ingenious, but want of space prevents us describing them. We
may point out, however, that many very good locking arrangements have
the defect of only locking the nut at certain points of a revolution,
say at every 30°. It will be noticed that the two arrangements which we
have described are not open to this objection.
EXERCISE 18.--Draw, full size, a plan, front elevation, and side
elevation of the arrangement of nuts shown in fig. 18, for a bolt
7/8 inch diameter.
EXERCISE 19.--Draw the plan and elevation of the nut and locking
arrangement shown in fig. 19. Make also an elevation looking in
the direction of the arrow. Scale 6 inches to a foot.
IV. KEYS.
_Keys_ are wedges, generally rectangular in section, but sometimes
circular; they are made of wrought iron or steel, and are used for
securing wheels, pulleys, cranks, &c., to shafts.
[Illustration: Fig. 20.]
Various sections of keys are shown in fig. 20. At (_a_) is the _hollow_
or _saddle key_. With this form of key it is not necessary to cut the
shaft in any way, but its holding power is small, and it is therefore
only used for light work. At (_b_) is the _key on a flat_, sometimes
called a _flat key_. The holding power of this key is much greater than
that of the saddle key. At (_c_) is the _sunk key_, a very secure and
very common form.
The part of the shaft upon which a key rests is called the _key bed_ or
_key way_, and the recess in the boss of the wheel or pulley into which
the key fits is called the _key way_; both are also called _key seats_.
With saddle, flat, and sunk keys the key bed is parallel to the axis of
the shaft; but the key way is deeper at one end than the other to
accommodate the taper of the key. The sides of the key are parallel.
The _round key_ or taper pin shown at (_d_) is in general only used for
wheels or cranks which have been previously shrunk on to their shafts or
forced on by great pressure. After the wheel or crank has been shrunk
on, a hole is drilled, half into the shaft and half into the wheel or
crank, to receive the pin.
When the point of a key is inaccessible the other end is provided with a
_gib head_ as shown at (_e_), to enable the key to be withdrawn.
A _sliding_ or _feather key_ secures a piece to a shaft so far as to
prevent the one from rotating without the other, but allows of relative
motion in the direction of the axis of the shaft. This form of key has
no taper, and it is secured to the piece carried by the shaft, but is
made a _sliding fit_ in the key way of the shaft. In one form of feather
key the part within the piece carried by the shaft is dovetailed as
shown at (_f_). In another form the key has a round projecting pin
forged upon it, which enters a corresponding hole as shown at (_g_). The
feather key may also be secured to the piece carried by the shaft by
means of one or more screws as shown at (_h_). The key way in the shaft
is made long enough to permit of the necessary sliding motion.
_Cone Keys._--These are sometimes fitted to pulleys, and are shown in
fig. 32, page 38. In this case the eye of the pulley is tapered and is
larger than the shaft. The space between the shaft and the boss of the
pulley is filled with three _saddle_ or _cone keys_. These keys are made
of cast iron and are all cast together, and before being divided the
casting is bored to fit the shaft and turned to fit the eye of the
pulley. By this arrangement of keys the same pulley may be fixed on
shafts of different diameters by using keys of different thicknesses;
also the pulley may be bored out large enough to pass over any boss
which may be forged on the shaft.
_Proportions of Keys._--The following rules are taken from Unwin's
'Machine Design,' pp. 142-43.
Diameter of eye of wheel, or boss of shaft = _d_.
Width of key = 3/4_d_ + 1/8.
Mean thickness of sunk key = 1/8_d_ + 1/8.
" key on flat = 1/16_d_ + 1/16.
The following table gives dimensions agreeing with average practice.
_Dimensions of Keys._
D = diameter of shaft.
B = breadth of key.
T = thickness of sunk key.
T_{1} = thickness of flat key, also = thickness of saddle key. Taper
of key 1/8 inch per foot of length, _i.e._ 1 in 96.
+---------------------------------------------------------------+
| D | 3/4 | 1 | 1-1/4 | 1-1/2 | 1-3/4 | 2 | 2-1/4 | 2-1/2 |
+-----+-----+-----+-------+-------+-------+-----+-------+-------+
| B | 5/16| 3/8 | 7/16 | 1/2 | 9/16 | 5/8 | 11/16 | 11/16 |
| T | 1/4 | 1/4 | 1/4 | 5/16 | 5/16 | 5/16| 3/8 | 3/8 |
|T_{1}| 3/16| 3/16| 3/16 | 3/16 | 1/4 | 1/4 | 1/4 | 5/16 |
+---------------------------------------------------------------+
+-------------------------------------------------------------------+
| D | 2-3/4 | 3 | 3-1/2 | 4 | 4-1/2 | 5 | 5-1/2 | 6 |
+-----+-------+-----+-------+-------+-------+-------+-------+-------+
| B | 3/4 | 7/8 | 1 | 1-1/8 | 1-1/4 | 1-3/8 | 1-1/2 | 1-5/8 |
| T | 3/8 | 7/16| 1/2 | 1/2 | 9/16 | 5/8 | 11/16 | 3/4 |
|T_{1}| 5/16 | 5/16| 3/8 | 7/16 | 1/2 | 1/2 | 9/16 | 5/8 |
+-------------------------------------------------------------------+
+-------------------------------------------------------+
| D | 7 | 8 | 9 | 10 | 11 | 12 |
+-----+-------+-------+-------+--------+--------+-------+
| B | 1-7/8 | 2-1/8 | 2-3/8 | 2-5/8 | 2-7/8 | 3-1/8 |
| T | 13/16 | 15/16 | 1 | 1-1/16 | 1-3/16 | 1-1/4 |
|T_{1}| 11/16 | 3/4 | 7/8 | 15/16 | 1-1/16 | 1-1/8 |
+-------------------------------------------------------+
V. SHAFTING.
Shafting is nearly always cylindrical and made of wrought iron or steel.
Cast iron is rarely used for shafting.
_Axles_ are shafts which are subjected to bending without twisting.
The parts of a shaft or axle which rest upon the bearings or supports
are called _journals_, _pivots_, or _collars_.
In journals the supporting pressure is at right angles to the axis of
the shaft, while in pivots and collars the pressure is parallel to that
axis.
Shafts may be solid or hollow. Hollow shafts are stronger than solid
shafts for the same weight of material. Thus a hollow shaft having an
external diameter of 10-1/4 inches and an internal diameter of 7 inches
would have about the same weight as a solid shaft of the same material
7-1/2 inches in diameter, but the former would have about double the
strength of the latter. Hollow shafts are also stiffer and yield less to
bending action than solid shafts, which in some cases, as in propeller
shafts, is an objection.
VI. SHAFT COUPLINGS.
For convenience of making and handling, shafts used for transmitting
power are generally made in lengths not exceeding 30 feet. These lengths
are connected by couplings, of which we give several examples.
[Illustration: FIGS. 21 and 22.]
_Solid_, _Box_, or _Muff Couplings._--One form of box coupling is shown
in fig. 21. Here the ends of the shafts to be connected butt against one
another, meeting at the centre of the box, which is made of cast iron.
The shafts are made to rotate as one by being secured to the box by two
wrought-iron or steel keys, both driven from the same end of the box. A
clearance space is left between the head of the forward key and the
point of the hind one, to facilitate the driving of them out, as then
only one key needs to be started at a time. Sometimes a single key the
whole length of the box is used, in which case it is necessary that the
key ways in the shafts be of exactly the same depth.
The half-lap coupling, introduced by Sir William Fairbairn, is shown in
fig. 22. In this form of box coupling the ends of the shafts overlap
within the box. It is evident that one shaft cannot rotate without the
other as long as the box remains over the lap. To keep the box in its
place it is fitted with a saddle key.
It will be noticed that the lap joint is sloped in such a way as to
prevent the two lengths of shaft from being pulled asunder by forces
acting in the direction of their length.
Half-lap couplings are not used for shafts above 5 inches in diameter.
It may here be pointed out that the half-lap coupling is expensive to
make, and is now not much used.
As shafts are weakened by cutting key ways in them, very often the ends
which carry couplings are enlarged in diameter, as shown in fig. 21, by
an amount equal to the thickness of the key. An objection to this
enlargement is that wheels and pulleys require either that their bosses
be bored out large enough to pass over it, or that they be split into
halves, which are bolted together after being placed on the shaft.
_Dimensions of Box Couplings._
D = diameter of shaft.
T = thickness of metal in box.
L = length of box for butt coupling.
L_{1} = length of box for lap coupling.
_l_ = length of lap.
D_{1} = diameter of shaft at lap.
+---------------------------------------------------------------+
| D | 1-1/2 | 2 | 2-1/2 | 3 | 3-1/2 | 4 |
+-------+--------+--------+---------+-------+----------+--------+
| T | 1-1/8 | 1-5/16 | 1-1/2 | 1-3/4 | 1-15/16 | 2-1/8 |
| L | 5-3/4 | 7 | 8-1/4 | 9-1/2 | 10-3/4 | 12 |
| L_{1} | 4-1/8 | 5-1/4 | 6-3/8 | 7-1/2 | 8-5/8 | 9-3/4 |
| _l_ | 1-7/16 | 1-7/8 | 2-5/16 | 2-3/4 | 3-3/16 | 3-5/8 |
| D_{2} | 2-5/16 | 3 | 3-11/16 | 4-3/8 | 5-1/16 | 5-3/4 |
+---------------------------------------------------------------+
+----------------------------------------------+
| D | 4-1/2 | 5 | 5-1/2 | 6 |
+-------+---------+--------+--------+----------+
| T | 2-5/16 | 2-1/2 | 2-3/4 | 2-15/16 |
| L | 13-1/4 | 14-1/2 | 15-3/4 | 17 |
| L_{1} | 10-7/8 | 12 | -- | -- |
| _l_ | 4-1/16 | 4-1/2 | -- | -- |
| D_{2} | 6-7/16 | 7-1/8 | -- | -- |
+----------------------------------------------+
Slope of lap 1 in 12.
EXERCISE 20: _Solid Butt Coupling._--From the above table of
dimensions make a longitudinal and a transverse section of a solid
butt coupling for a shaft 2-1/2 inches in diameter. Scale 6 inches
to a foot.
EXERCISE 21: _Fairbairn's Half-Lap Coupling._--Make the same views
as in the last exercise of a half-lap coupling for a 3-inch shaft
to the dimensions in the above table. Scale 6 inches to a foot.
_Flange Couplings._--The form of coupling used for the shafts of marine
engines is shown in fig. 23. The ends of the different lengths of shaft
have flanges forged on them, which are turned along with the shaft.
These flanges butt against one another, and are connected by bolts.
These bolts may be parallel or tapered; generally they are tapered. A
parallel bolt must have a head, but a tapered bolt will act without one.
In fig. 23 the bolts are tapered, and also provided with heads. In fig.
14, page 17, is shown a tapered bolt without a head. The variation of
diameter in tapered bolts is 3/8 of an inch per foot of length.
[Illustration: FIG. 23.]
Sometimes a projection is formed on the centre of one flange which fits
into a corresponding recess in the centre of the other, for the purpose
of ensuring the shafts being in line.
Occasionally a cross-key is fitted in between the flanges, being sunk
half into each, for the purpose of diminishing the shearing action on
the bolts.
EXERCISE 22: _Marine Coupling._--Draw the elevation and section of
the coupling shown in fig. 23; also an elevation looking in the
direction of the arrow. Scale 3 inches to a foot.
The following table gives the dimensions of a few marine couplings taken
from actual practice.
_Examples of Marine Couplings._
+--------------------------------------------------------------------+
| Diameter of shaft |2-3/8 | 9-3/4 | 12-7/8 |16-1/2 | 22-1/2 | 23 |
+--------------------+------+-------+--------+-------+--------+------+
|Diameter of flange | 6 | 19 | 24 | 32 | 35 | 38 |
|Thickness of flange | 1 | 2-3/4 | 3-1/8 | 4-1/4 | 6 | 5 |
|Diameter of bolts | 3/4 | 2-3/4 | 2-11/16| 3-1/2 | 4-1/4 | 4-1/4|
|Number of bolts | 3 | 6 | 6 | 8 | 9 | 8 |
|Diameter of bolt | | | | | | |
| circle |4-1/8 | 14-1/8|18-13/16| 25 | 28-3/4 |30-3/8|
+--------------------------------------------------------------------+
All the above dimensions are in inches.
EXERCISE 23.--Select one of the couplings from the above table, and
make the necessary working drawings for it to a suitable scale.
The cast-iron flange coupling is shown in fig. 24. In this kind of
coupling a cast-iron centre or boss provided with a flange is secured to
the end of each shaft by a sunk key driven from the face of the flange.
These flanges are then connected by bolts and nuts as in the marine
coupling.
To ensure the shafts being in line the end of one projects into the
flange of the other.
In order that the face of each flange may be exactly perpendicular to
the axis of the shaft they should be 'faced' in the lathe, after being
keyed on to the shaft.
If the coupling is in an exposed position, where the nuts and bolt-heads
would be liable to catch the clothes of workmen or an idle driving band
which might come in the way, the flanges should be made thicker, and be
provided with recesses for the nuts and bolt-heads.
[Illustration: FIG. 24.]
_Dimensions of Cast-iron Flange Couplings._
+--------------------------------------------------------------------+
| |Diameter| | |Depth | |Diameter|Diameter|
|Diameter| of |Thickness|Diameter| at |Number| of | of bolt|
|of shaft| flange |of flange| of boss| boss | of | bolts | circle |
| D | F | T | B | L | bolts| d | C |
+--------|--------|---------|--------|------|------|--------|--------+
| 1-1/2 | 7-1/4 | 7/8 | 3-1/2 |2-5/8 | 3 |5/8 | 5-1/2 |
| 2 | 8-7/8 | 1-1/16 | 4-3/8 |3-3/16| 4 | 3/4| 6-3/4 |
| 2-1/2 | 10-5/8 | 1-1/4 | 5-5/16 |3-3/4 | 4 |7/8 | 8-1/8 |
| 3 | 12-3/8 | 1-7/16 | 6-1/4 |4-5/16| 4 | 1 | 9-1/2 |
| 3-1/2 | 13-1/8 | 1-5/8 | 7-1/8 |4-7/8 | 4 | 1 |10-5/16 |
| 4 | 14 | 1-3/4 | 8 |5-7/16| 6 | 1 |11-1/4 |
| 4-1/2 | 15-5/8 | 2 | 8-7/8 |6 | 6 |1-1/8 |12-1/2 |
| 5 | 17-3/8 | 2-1/8 | 9-13/16|6-5/8 | 6 | 1-1/4|13-13/16|
| 5-1/2 | 18-1/4 | 2-5/16 |10-3/4 |7-1/4 | 6 |1-1/4 |14-3/4 |
| 6 | 19-7/8 | 2-1/2 |11-5/8 |7-3/4 | 6 | 1-3/8| 16 |
+--------------------------------------------------------------------+
The projection of the shaft _p_ varies from 1/4 inch in the small shafts
to 1/2 inch in the large ones.
EXERCISE 24: _Cast-iron Flange Coupling._--Draw the views shown in
fig. 24 of a cast-iron flange coupling, for a shaft 4-1/2 inches in
diameter, to the dimensions given in the above table. Scale 4 inches
to a foot.
VII. BEARINGS FOR SHAFTS.
An example of a very simple form of bearing is shown in fig. 25, which
represents a brake shaft carrier of a locomotive tender. The bearing in
this example is made of cast iron and in one piece. Through the
oval-shaped flange two bolts pass for attaching the bearing to the
wrought-iron framing of the tender. With this form of bearing there is
no adjustment for wear, so that when it becomes worn it must be renewed.
[Illustration: FIG. 25.]
EXERCISE 25: _Brake Shaft Carrier._--Draw the elevation and
sectional plan of the bearing shown in fig. 25. Draw also a
vertical section through the axis. The latter view to be projected
from the first elevation. Scale 6 inches to a foot.
_Pillow Block_, _Plummer Block_, or _Pedestal_.--The ordinary form of
plummer block is represented in fig. 26. A is the block proper, B the
sole through which pass the holding-down bolts. C is the cap. Between
the block and the cap is the brass bush, which is in halves, called
_brasses_ or _steps_. The bed for the steps in this example is
cylindrical, and is prepared by the easy process of boring. The steps
are not supported throughout their whole length, but at their ends only
where fitting strips are provided as shown. As the wear on a step is
generally greatest at the bottom, it is made thicker there than at the
sides, except where the fitting strips come in. To prevent the steps
turning within the block they are generally furnished with lugs, which
enter corresponding recesses in the block and cover.
[Illustration: FIG. 26]
In the block illustrated the journal is lubricated by a _needle
lubricator_; this consists of an inverted glass bottle fitted with a
wood stopper, through a hole in which passes a piece of wire, which has
one end in the oil within the bottle, and the other resting on the
journal of the shaft. The wire or needle does not fill the hole in the
stopper, but if the needle is kept from vibrating the oil does not
escape owing to capillary attraction. When, however, the shaft rotates,
the needle begins to vibrate, and the oil runs down slowly on to the
journal; oil is therefore only used when the shaft is running.
EXERCISE 26: _Pillow Block for a Four-inch Shaft._--Draw the views
shown of this block in fig. 26. Make also separate drawings, full
size, of one of the steps. Scale 6 inches to a foot.
_Proportions of Pillow Blocks._--The following rules may be used for
proportioning pillow blocks for shafts up to 8 inches diameter. It
should be remembered that the proportions used by different makers vary
considerably, but the following rules represent average practice.
Diameter of journal = _d_.
Length of journal = _l_.
Height to centre = 1.05_d_ + .5.
Length of base = 3.6_d_ + 5.
Width of base = .8_l_.
" block = .7_l_.
Thickness of base = .3_d_ + .3.
" cap = .3_d_ + .4.
Diameter of bolts = .25_d_ + .25.
Distance between centres of cap bolts = 1.6_d_ + 1.5.
" " base bolts = 2.7_d_ + 4.2.
Thickness of step at bottom = _t_ = .09_d_ + .15.
" " sides = 3/4 _t_.
The length of the journal varies very much in different cases, and
depends upon the speed of the shaft, the load which it carries, the
workmanship of the journal and bearing, and the method of lubrication.
For ordinary shafting one rule is to make _l_ = _d_ + 1. Some makers use
the rule _l_ = 1.5_d_; others make _l_ = 2_d_.
EXERCISE 27: _Design for Pillow Block._--Make the necessary
working drawings for a pillow block for a shaft 5 inches in
diameter, and having a journal 7 inches long.
[Illustration: FIG. 27.]
_Brackets._--When a pillow block has to be fixed to a wall or column a
bracket such as that shown in figs. 27 and 28 may be used. The pillow
block rests between the _joggles_ A A, and is bolted down to the bracket
and secured in addition with keys at the ends of the base of the block,
in the same manner as is shown, for the attachment of the bracket to
the column.
EXERCISE 28: _Pillar Bracket._--Fig. 27 shows a side elevation and
part horizontal section, and fig. 28 shows an end elevation of a
pillar bracket for carrying a pillow block for a 3-inch shaft.
Draw these views _properly projected from one another_, showing
the pillow block, which is to be proportioned by the rules given
on page 32. Draw also a plan of the whole. Scale 4 inches to a
foot.
[Illustration: FIG. 28.]
_Hangers._--When a shaft is suspended from a ceiling it is carried by
hangers, one form of which is shown in fig. 29, and which will be
readily understood. The cap of the bearing, it will be noticed, is
secured by means of a bolt, and also by a square key.
EXERCISE 29: _Shaft Hanger._--Draw the two elevations shown in
fig. 29, and also a sectional plan. The section to be taken at a
point 5 inches above the centre of the shaft. Scale 6 inches to a
foot.
_Wall Boxes._--In passing from one part of a building to another a shaft
may have to pass through a wall. In that case a neat appearance is given
to the opening and a suitable support obtained for a pillow block by
building into the wall a _wall box_, one form of which is shown in fig.
30.
EXERCISE 30: _Wall Box._--Draw the views of the wall box shown in
fig. 30, and also a sectional plan; the plane of section to pass
through the box a little above the joggles for the pillow block.
Scale 3 inches to a foot.
[Illustration: FIG. 29.]
[Illustration: FIG. 30.]
VIII. PULLEYS.
_Velocity Ratio in Belt Gearing._--Let two pulleys A and B be connected
by a belt, and let their diameters be D_{1} and D_{2}; and let their
speeds, in revolutions per minute, be N_{1} and N_{2} respectively. If
there is no slipping, the speeds of the rims of the pulleys will be the
same as that of the belt, and will therefore be equal. Now the speed of
the rim of A is evidently = D_{1} × 3.1416 × N_{1}; while the speed of
the rim of B is = D_{2} × 3.1416 × N_{2}. Hence D_{1} × 3.1416 × N_{1} =
D_{2} × 3.1416 × N_{2}, and therefore
N_{1} D_{2}
----- = -----.
N_{2} D_{1}
_Pulleys for Flat Bands._--In cross section the rim of a pulley for
carrying a flat band is generally curved as shown in figs. 31 and 32,
but very often the cross section is straight. The curved cross section
of the rim tends to keep the band from coming off as long as the pulley
is rotating. Sometimes the rim of the pulley is provided with flanges
which keep the band from falling off.
Pulleys are generally made entirely of cast iron, but a great many
pulleys are now made in which the centre or nave only is of cast iron,
the arms being of wrought iron cast into the nave, while the rim is of
wrought sheet iron.
The arms of pulleys when made of wrought iron are invariably straight,
but when made of cast iron they are very often curved. In fig. 31, which
shows an arrangement of two cast-iron pulleys, the arms are straight;
while in fig. 32, which shows another cast-iron pulley, the arms are
curved. Through unequal cooling, and therefore unequal contraction of a
cast-iron, pulley in the mould, the arms are generally in a state of
tension or compression; and if the arms are straight they are very
unyielding, so that the result of this initial stress is often the
breaking of an arm, or of the rim where it joins an arm. With the curved
arm, however, its shape permits it to yield, and thus cause a diminution
of the stress due to unequal contraction.
The cross section of the arms of cast-iron pulleys is generally
elliptical.
[Illustration: Fig. 31.]
EXERCISE 31: _Fast and Loose Pulleys_.--Fig. 31 shows an
arrangement of fast and loose pulleys. A is the fast pulley,
secured to the shaft C by a sunk key; B is the loose pulley, which
turns freely upon the shaft. The loose pulley is prevented from
coming off by a collar D, which is secured to the shaft by a
tapered pin as shown. The nave or boss of the loose pulley is here
fitted with a brass liner, which may be renewed when it becomes
too much worn. Draw the elevations shown, completing the left-hand
one. Scale 6 inches to a foot.
By the above arrangement of pulleys a machine may be stopped or
set in motion at pleasure. When the driving band is on the loose
pulley the machine is at rest, and when it is on the fast pulley
the machine is in motion. The driving band is shifted from the one
pulley to the other by pressing on that side of the band which is
advancing towards the pulleys.
[Illustration: FIG. 32.]
EXERCISE 32: _Cast-iron Pulley with Curved Arms and Cone
Keys_.--Draw a complete side elevation and a complete cross
section of the pulley represented in fig. 32 to a scale of 3
inches to a foot. In drawing the side elevation of the arms first
draw the centre lines as shown; next draw three circles for each
arm, one at each end and one in the middle; the centres of these
circles being on the centre line of the arm, and their diameters
equal to the widths of the arm at the ends and at the middle
respectively. Arcs of circles are then drawn to touch these three
circles. The centres and radii of these arcs may be found by
trial. The cone keys for securing the pulley to the shaft were
described on p. 23.
_Pulleys for Ropes_.--Ropes made of hemp are now extensively used for
transmitting power. These ropes vary in diameter from 1 inch to 2
inches, and are run at a speed of about 4,500 feet per minute. The
pulleys for these ropes are made of cast iron, and have their rims
grooved as shown in fig. 33, which is a cross section of the rim of a
pulley carrying three ropes. The angle of the V is usually 45°, and the
rope rests on the sides of the groove, and not on the bottom, so that
it is wedged in, and has therefore a good hold of the pulley. The
diameter of the pulley should not be less than 30 times the diameter of
the rope. Two pulleys connected by ropes should not be less than thirty
feet apart from centre to centre, but this distance may be as much as
100 feet.
[Illustration: FIG. 33.]
EXERCISE 33: _Section of Rim of Rope Pulley._--Draw, half size,
the section of the rim of a rope pulley shown in fig. 33.
IX. TOOTHED WHEELS.
_Pitch Surfaces of Spur Wheels._--Let two smooth rollers be placed in
contact with their axes parallel, and let one of them rotate about its
axis; then if there is no slipping the other roller will rotate in the
opposite direction with the same surface velocity; and if D_{1}, D_{2}
be the diameters of the rollers, and N_{1}, N_{2} their speeds in
revolutions per minute, it follows as in belt gearing that--
N_{1} D_{2}
----- = -----.
N_{2} D_{1}
If there be considerable resistance to the motion of the follower
slipping may take place, and it may stop. To prevent this the rollers
may be provided with teeth; then they become _spur wheels_; and if the
teeth be so shaped that the ratio of the speeds of the toothed rollers
at any instant is the same as that of the smooth rollers, the surfaces
of the latter are called the _pitch surfaces_ of the former.
_Pitch Circle._--A section of the pitch surface of a toothed wheel by a
plane perpendicular to its axis is a circle, and is called a _pitch
circle_. We may also say that the pitch circle is the edge of the pitch
surface. The pitch circle is generally traced on the side of a toothed
wheel, and is rather nearer the points of the teeth than the roots.
_Pitch of Teeth._--The distance from the centre of one tooth to the
centre of the next, or from the front of one to the front of the next,
_measured at the pitch circle_, is called the _pitch of the teeth_. If D
be the diameter of the pitch circle of a wheel, _n_ the number of teeth,
and _p_ the pitch of the teeth, then D × 3.1416 = _n_ × _p_.
[Illustration: FIG. 34.]
By the diameter of a wheel is meant the diameter of its pitch circle.
_Form and Proportions of Teeth._--The ordinary form of wheel teeth is
shown in fig. 34. The curves of the teeth should be cycloidal curves,
although they are generally drawn in as arcs of circles. It does not
fall within the scope of this work to discuss the correct forms of wheel
teeth. The student will find the theory of the teeth of wheels clearly
and fully explained in Goodeve's 'Elements of Mechanism,' and in Unwin's
'Machine Design.'
The following proportions for the teeth of ordinary toothed wheels may
be taken as representing average practice:--
Pitch of teeth = _p_ = arc _a b c_ (fig. 34).
Thickness of tooth = _b c_ = .48_p_.
Width of space = _a b_ = .52_p_.
Total height of tooth = _h_ = .7_p_.
Height of tooth above pitch line = _k_ = .3_p_.
Depth of tooth below pitch line = _l_ = .4_p_.
Width of tooth = 2_p_ to 3_p_.
EXERCISE 34: _Spur Wheel._--Fig. 35 shows the elevation and
sectional plan of a portion of a cast-iron spur wheel. The diameter
of the pitch circle is 23-7/8 inches, and the pitch of the teeth is
1-1/2 inches, so that there will be 50 teeth in the wheel. The
wheel has six arms. Draw a complete elevation of the wheel and a
half sectional plan, also a half-plan without any section. Draw
also a cross section of one arm. Scale 4 inches to a foot.
[Illustration: FIG. 35.]
_Mortise Wheels._--When two wheels gearing together run at a high speed
the teeth of one are made of wood. These teeth, or cogs, as they are
generally called, have tenons formed on them, which fit into mortises in
the rim of the wheel. This wheel with the wooden teeth is called a
_mortise wheel_. An example of a mortise wheel is shown in fig. 36.
[Illustration: FIG. 36.]
_Bevil Wheels._--In bevil wheels the pitch surfaces are parts of cones.
Bevil wheels are used to connect shafts which are inclined to one
another, whereas spur wheels are used to connect parallel shafts. In
fig. 36 is shown a pair of bevil wheels in gear, one of them being a
mortise wheel. At (_a_) is a separate drawing, to a smaller scale, of
the pitch cones. The pitch cones are shown on the drawing of the
complete wheels by dotted lines.
The diameters of bevil wheels are the diameters of the bases of their
pitch cones.
EXERCISE 35: _Pair of Bevil Wheels._--Draw the sectional elevation
of the bevil wheels shown in gear in fig. 36. Commence by drawing
the centre lines of the shafts, which in this example are at right
angles to one another; then draw the pitch cones shown by dotted
lines. Next put in the teeth which come into the plane of the
section, then complete the sections of the wheels. The pinion or
smaller wheel has 25 teeth, and the wheel has 50 teeth, which makes
the pitch a little over 3 inches. Each tooth of the mortise wheel
is secured as shown by an iron pin 5/16 inch diameter. Scale 3
inches to a foot.
X. CRANKS AND CRANKED SHAFTS.
The most important application of the crank is in the steam-engine,
where the reciprocating rectilineal motion of the piston is converted
into the rotary motion of the crank-shaft by means of the crank and
connecting rod.
At one time steam-engine cranks were largely made of cast iron, now they
are always made of wrought iron or steel. The crank is either forged in
one piece with the shaft, or it is made separately and then keyed to it.
_Overhung Crank._--Fig. 37 shows a wrought-iron overhung crank. A is the
crank-shaft, B the crank arm, provided at one end with a boss C, which
is bored out to fit the shaft; at the other end of the crank arm is a
boss D, which is bored out to receive the crank-pin E, which works in
one end of the connecting rod. The crank is secured to the shaft by the
sunk key F. It is also good practice to _shrink_ the crank on to the
shaft. The process of shrinking consists of boring out the crank a
little smaller than the shaft, and then heating it, which causes it to
expand sufficiently to go on to the shaft. As the crank cools, it
shrinks and grips the shaft firmly. The crank may also be shrunk on to
the crank-pin, the latter being then riveted over as shown in fig. 37.
[Illustration: FIG. 37.]
A good plan to adopt in preference to the shrinking process is to force
the parts together by hydraulic pressure. This method is adopted for
placing locomotive wheels on their axles, and for putting in crank-pins.
As to the amount of pressure to be used, the practice is to allow a
force of 10 tons for every inch of diameter of the pin, axle, or shaft.
Instead of being riveted in, the crank pin may be prolonged and screwed,
and fitted with a nut. Another plan is to put a cotter through the crank
and the crank-pin.
The distance from the centre of the crank-shaft to the centre of the
crank-pin is called the radius of the crank. The _throw_ of the crank is
twice the radius. In a direct-acting engine the throw of the crank is
equal to the stroke of the piston.
EXERCISE 36: _Wrought-iron Overhung Crank._--Draw the two
elevations shown in fig. 37, also a plan. Scale 1-1/2 inches
to a foot.
_Proportions of Overhung Cranks._
D = diameter of shaft.
_d_ = " crank-pin.
Length of large boss = .9 D.
Diameter " = 1.8 D.
Length of small boss = 1.1 _d_.
Diameter " = 1.8 _d_.
Width of crank arm at centre of shaft = 1.3 D.
" " crank-pin = 1.5 _d_.
The thickness of the crank arm may be roughly taken as = .7 D.
EXERCISE 37.--Design a wrought-iron crank for an engine having a
stroke of 4 feet. The crank-shaft is 9 inches in diameter, and
the crank-pin is 4-3/4 inches in diameter and 6-1/2 inches long.
[Illustration: FIG. 38.]
_Locomotive Cranked Axle._--As an example of a cranked shaft we take the
cranked axle for a locomotive with inside cylinders shown in fig. 38;
here the crank and shaft or axle are forged in one piece. A is the wheel
seat, B the journal, C the crank-pin, and D and E the crank arms. Only
one half of the axle is shown in fig. 38, but the other half is exactly
the same. The cranks on the two halves are, however, at right angles to
one another. The ends of the crank arms are turned in the lathe, the
crank-pin ends being turned at the same time as the axle, and the other
ends at the same time as the crank-pin. This consideration determines
the centres for the arcs shown in the end view.
EXERCISE 38.--Draw to a scale of 2 inches to a foot the side and
end elevations of the locomotive cranked axle partly shown in
fig. 38. The distance between the centre lines of the cylinders
is 2 feet.
[Illustration: FIG. 39.]
_Built-up Cranks._--The form of cranked shaft shown in fig. 38 is
largely used for marine engines, but for the very powerful engines now
fitted in large ships this design of shaft is very unreliable, the
built-up crank shown in fig. 39 being preferred, although it is much
heavier than the other. It will be seen from the figure that the shaft,
crank arms, and crank-pin are made separately. The arms are shrunk on to
the pin and the shaft, and secured to the latter by sunk keys. These
heavy shafts and cranks are generally made of steel.
EXERCISE 39.--Keeping to the dimensions marked in fig. 39, draw
the views there shown of a built-up crank-shaft for a marine
engine. Scale 3/4 inch to a foot.
XI. ECCENTRICS.
The _eccentric_ is a particular form of crank, being a crank in which
the crank-pin is large enough to embrace the crank-shaft. In the
eccentric what corresponds to the crank-pin is called the sheave or
pulley. The advantage which an eccentric possesses over a crank is that
the shaft does not require to be divided at the point where the
eccentric is put on. The crank, however, has this advantage over the
eccentric, namely, that it can be used for converting circular into
reciprocating motion, or _vice versâ_, while the eccentric can only be
used for converting circular into reciprocating motion. This is owing to
the great leverage at which the friction of the eccentric acts.
The chief application of the eccentric is in the steam-engine, where it
is used for working the valve gear.
To permit of the sheave being placed on the shaft without going over the
end (which could not be done at all in the case of a cranked axle, and
would be a troublesome operation in most cases) it is generally made in
two pieces, as shown in fig. 40, which represents one of the eccentrics
of a locomotive. The two parts of the sheave are connected by two cotter
bolts. The part which embraces the sheave is called the eccentric strap,
and corresponds to, and is, in fact, a connecting rod end: the rod
proceeding from this is called the eccentric rod.
The distance from the centre of the sheave to the centre of the shaft is
called the _radius_ or _eccentricity_ of the eccentric. The _throw_ is
twice the eccentricity.
The sheave is generally made of cast iron. The strap may be of brass,
cast iron, or wrought iron; when the strap is made of wrought iron it is
commonly lined with brass.
[Illustration: FIG. 40.]
EXERCISE 40: _Locomotive Eccentric._--In fig. 40 D E is the
sheave, F H the strap, and K the eccentric rod. The sheave and
strap are made of cast iron, and the eccentric rod is made of
wrought iron. (_a_) is a vertical cross section through the
oil-box of the strap; (_b_) is a plan of the end of the eccentric
rod and part of the strap. All the nuts are locked by means of
cotters. Draw first the elevation, partly in section as shown.
Next draw two end elevations, one looking each way. Afterwards
draw a horizontal section through the centre, and also a plan.
Scale 4 inches to a foot.
XII. CONNECTING RODS.
The most familiar example of the use of a connecting rod is in the
steam-engine, where it is used to connect the rotating crank with the
reciprocating piston. The rod itself is made of wrought iron or steel,
and is generally circular or rectangular in section. The ends of the rod
are fitted with steps, which are held together in a variety of ways.
_Strap End._--A form of connecting rod end, which is not so common as it
used to be, is shown in fig. 41. At (_a_) is shown a longitudinal
section with all the parts put together, while at (_b_), (_c_), _(d)_
and (_e_) the details are shown separately. A B is the end of the rod
which butts against the brass bush C D, which is in two pieces. A
_strap_ E passes round the bush and on to the end of the rod as shown.
The arms of the strap have rectangular holes in them, which are not
quite opposite a similar hole in the rod when the parts are put
together. If a wedge or _cotter_ F be driven into these three holes they
will tend to come into line, and the parts of the bush will be pressed
together. To prevent the cotter opening out the strap, and to increase
the sliding surface, a _gib_ H is introduced. The gib is provided with
horns at its ends to keep it in its place. Sometimes two gibs are used,
one on each side of the cotter; this makes the sliding surface on both
sides of the cotter the same. The cotter is secured by a set screw K.
The unsectioned portion of fig. (_a_) to the right of the gib, or to the
left of the cotter, is called the _clearance_ or _draught._
[Illustration: FIG. 41.]
EXERCISE 41: _Connecting Rod End._--Make the following views of
the connecting rod end illustrated by fig. 41. First, a vertical
section, the same as shown at (_a_). Second, a horizontal section.
Third, side elevation. Fourth, a plan. Or the first and third
views may be combined in a half vertical section and half
elevation; and the second and fourth views may be combined in
a half horizontal section and half plan.
All the dimensions are to be taken from the detail drawings (_b_),
(_c_), (_d_), and (_e_), _but the details need not be drawn
separately_. The brass bush is shown at (_d_) by half elevation,
half vertical section, half plan, and half horizontal section.
The draught or clearance is 7-16ths of an inch.
_Box End._--At (_a_), fig. 42, is shown what is known as a box end for a
connecting rod. The part which corresponds to the loose strap in the
last example is here forged in one piece with the connecting rod. In
this form the brass bush is provided with a flange all round on one
side, but on the opposite side the flange is omitted except at one end;
this is to allow of the bush being placed within the end of the rod. The
construction of the bush will be understood by reference to the sketch
shown at (_b_). The bush is in two parts, which are pressed tightly
together by means of a cotter. This cotter is prevented from slackening
back by two set screws. Each set screw is cut off square at the point,
and presses on the flat bottom of a very shallow groove cut on the side
of the cotter.
The top, bottom, and ends of this box end are turned in the lathe at the
same time as the rod itself; this accounts for the curved sections of
these parts.
It is clear from the construction of a box end that it is only suitable
for an overhung crank.
EXERCISE 42: _Locomotive Connecting Rod._--In fig. 42 is shown a
connecting rod for an outside cylinder locomotive. (_a_) is the
crank-pin end, and (_c_) the cross-head end. The end (_a_) has just
been described under the head 'box end.' We may just add that in
this particular example the brass bush is lined with white metal as
shown, and that the construction of the oil-box is the same as that
on the coupling rod end shown in fig. 44. The end (_c_) is forked,
and through the prongs of the fork passes the cross-head pin, of
which a separate dimensioned drawing is shown at (_d_). Observe
that the tapered parts A and B of this pin are parts of the same
cone. The rotation of the pin is prevented by a small key as shown.
The cross-head pin need not be drawn separately, and the isometric
projection of the bush at (_b_) may be omitted, but all the other
views shown are to be drawn to a scale of 6 inches to a foot.
_Marine Connecting Rod._--The form of connecting rod shown in fig. 43 is
that used in marine engines, but it is also used extensively in land
engines. A B is the crank-pin end, and C the cross-head end. The end A B
is forged in one piece, and after it is turned, planed, and bored it is
slotted across, so as to cut off the cap A. The parts A and B are held
together by two bolts as shown. This end of the rod is fitted with brass
steps, which are lined with white metal. The cross-head end is forked,
and through the prongs of the fork passes a pin D, which also passes
through the cross-head, which is forged on to the piston rod or attached
to it in some other way.
[Illustration: FIG. 42.]
[Illustration: FIG. 43.]
EXERCISE 43: _Marine Connecting Rod._--Draw all the views shown in
fig. 43 of one form of marine connecting rod. For detail drawings
of the locking arrangement for the nuts see fig. 19, page 21. Scale
4 inches to a foot.
_Coupling Rods._--A rod used to transmit the motion of one crank to
another is called a _coupling rod_. A familiar example of the use of
coupling rods will be found in the locomotive. Coupling rods are made of
wrought iron or steel, and are generally of rectangular section. The
ends are now generally made solid and lined with solid brass bushes,
_without any adjustment for wear_. This form of coupling rod end is
found to answer very well in locomotive practice where the workmanship
and arrangements for lubrication are excellent. When the brass bush
becomes worn it is replaced by a new one.
Fig. 44 shows an example of a locomotive coupling rod end for an outside
cylinder engine. In this case it is desirable to have the crank-pin
bearings for the coupling rods as short as possible, for a connecting
rod and coupling rod in this kind of engine work side by side on the
same crank-pin, which, being overhung, should be as short as convenient
for the sake of strength. The requisite bearing surface is obtained by
having a pin of large diameter. The brass bush is prevented from
rotating by means of the square key shown. The oil-box is cut out of the
solid, and has a wrought-iron cover slightly dovetailed at the edges.
This cover fits into a check round the top inner edge of the box, which
is originally parallel, but is made to close on the dovetailed edges of
the cover by riveting. A hole in the centre of this cover, which gives
access to the oil-box, is fitted with a screwed brass plug. The brass
plug has a screwed hole in the centre, through which oil may be
introduced to the box. Dust is kept out of the oil-box by screwing into
the hole in the brass plug a common cork. The oil is carried slowly but
regularly from the oil-box over to the bearing by a piece of cotton
wick.
[Illustration: FIG. 44.]
EXERCISE 44: _Coupling Rod End._--Draw first the side elevation and
plan, each partly in section as shown in fig. 44. Then instead of
the view to the left, which is an end elevation partly in section,
draw a complete end elevation looking to the right, and also a
complete vertical cross section through the centre of the bearing.
Scale 6 inches to a foot.
XIII. CROSS-HEADS.
An example of a steam-engine cross-head is shown in fig. 45. A is the
end of the piston rod which has forged upon it the cross-head B. The
cross-head pin shown at (_d_), fig. 42, and to which the connecting rod
is attached, works in the bearing C. Projecting pieces D, forged on the
top and bottom of the cross-head, carry the slide blocks E which work on
the slide bars, and thus guide the motion of the piston rod.
[Illustration: FIG. 45.]
EXERCISE 45: _Locomotive Cross-head._--In fig. 45 are shown side
and end elevations, partly in section, of the cross-head and slide
blocks for an outside cylinder locomotive. Draw these views half
size, showing also on the end elevation the cross-head pin and a
vertical section of the connecting rod end from fig. 42. The bush
in the cross-head which forms the bearing for the cross-head pin is
of wrought iron, case-hardened, and is prevented from rotating by
the key shown. The cross-head is of wrought iron, and the slide
blocks are of cast iron, and are fitted with white metal strips as
shown. A short brass tube leads oil from the upper slide block into
a hole in the cross-head as shown, which carries it to a slot in
the bush which distributes it over the cross-head pin.
XIV. PISTONS.
A _piston_ is generally a cylindrical piece which slides backwards and
forwards inside a hollow cylinder. The piston may be moved by the action
of fluid pressure upon it as in a steam-engine, or it may be used to
give motion to a fluid as in a pump.
A piston is usually attached to a rod, called a _piston rod_, which
passes through the end of the cylinder inside which the piston works,
and which serves to transmit the motion of the piston to some piece
outside the cylinder, or _vice versâ_.
[Illustration: FIG. 46.]
A _plunger_ is a piston made in one piece with its piston rod, the
piston and the rod being of the same diameter.
A piston which is provided with one or more valves which allow the
fluid to pass through it from one side to the other is called a
_bucket_.
_Simple Piston._--The simplest form of piston is a plain cylinder
fitting accurately another, inside which it moves. Such a piston works
with very little friction, but as there is no adjustment for wear, such
a piston is not suitable for a high fluid pressure if it has to work
constantly. This simple form of piston is used in the steam-engine
indicator, and also in pumps.
Fig. 46 shows the piston of the circulation pump of a marine engine.
A is the cast-iron casing or barrel of the pump; B is a brass liner
fitting tightly into the former at its ends, and secured by eight
screwed Muntz metal pins C, four at each end; D is the piston, which is
made of brass, and is attached to a Muntz metal piston rod E. The liner
is bored out smooth and true from end to end, and the piston is turned
so as to be a sliding fit to the liner. The wear in this form of piston
is diminished by making the rubbing surface large.
EXERCISE 46: _Piston for Circulating Pump._--Draw the vertical
sectional elevation of the piston, &c., shown in fig. 46, also a
half plan and half horizontal section through the centre. Scale 4
inches to a foot.
_Pump Bucket._--The next form of piston which we illustrate is shown in
fig. 47. This represents the air-pump bucket of a marine engine. The
bucket is made of brass, and is provided with six india-rubber disc
valves. The rod is in this case made of Muntz metal. Air-pump rods for
marine engines are very often made of wrought iron cased with brass. It
will be observed that there is a wide groove around the bucket, which is
filled with hempen rope or gasket. This gasket forms an elastic packing
which prevents leakage. This is an old-fashioned form of packing, and is
now only used for pump buckets.
[Illustration: FIG. 47.]
EXERCISE 47: _Air-pump Bucket._--Draw the sectional elevation of
the air-pump bucket shown in fig. 47. Also draw a half plan looking
downwards and a half plan looking upwards. Scale 4 inches to a
foot.
_Ramsbottom's Packing._--The form of packing used in the air-pump
bucket, fig. 47, is not suitable for steam pistons. For the latter the
packing is now always metallic. The simplest form of metallic packing is
that known as Ramsbottom's. This form is very largely used for
locomotive pistons, and for small pistons in many kinds of engines
besides. A locomotive piston for an 18-inch cylinder with Ramsbottom's
packing is shown in fig. 48. The particular piston there illustrated is
made of brass, and is secured to a wrought-iron piston rod by a brass
nut. Two circumferential grooves of rectangular section are turned out
of the piston, and into these fit two corresponding rings, which may be
of brass, cast iron, or steel. In this example the rings are of cast
iron. These rings are first turned a little larger in diameter than the
bore of the cylinder (in this example 1/2 inch), and then sprung over
the piston into the groves prepared for them. Their own elasticity
causes the rings to press outwards on the cylinder. At the point where a
ring is split a leakage of steam will take place, but with quick-running
pistons this leakage is unimportant. The points where the rings are cut
should be placed diametrically opposite, so as to diminish the leakage
of steam.
[Illustration: FIG. 48.]
EXERCISE 48: _Locomotive Piston._--A part elevation and part
section of a locomotive piston, for a cylinder having a bore 18
inches in diameter, is shown in fig. 48. Draw this, and also a view
looking on the nut in the direction of the axis of the piston rod.
Scale 6 inches to a foot.
_Note._--The reason why the part of the piston rod within the
piston has such a quick taper is that the piston has to be taken
off the rod while it is in the cylinder. The cross-head being
forged on the end of the piston rod prevents the piston and piston
rod being withdrawn together.
_Large Pistons._--Pistons of large diameter are generally provided with
two cast-iron packing rings placed within the same groove. These rings
are pressed outwards against the cylinder, and also against the sides of
the groove by one or more springs. One form of this packing
(Lancaster's) is shown in fig. 49. Here one spring only is used, and it
is first made a straight spiral spring, and then bent round and its ends
united. The action of the spring will be clearly understood from the
illustration. For the purpose of admitting the packing rings the piston
is divided into two parts, one the piston proper, and the other the
_junk ring_. In fig. 49, A is the junk ring, which is secured to the
piston by means of bolts as shown.
[Illustration: FIG. 49.]
EXERCISE 49: _Marine Engine Piston._--The piston illustrated by
fig. 49 is for the high-pressure cylinder of a marine engine. The
piston, junk ring, and packing rings are of cast iron. The piston
rod and nut are of wrought iron, so also are the junk ring bolts.
The nuts for the latter are of brass. The spiral spring is made
from steel wire 3/8 inch diameter. An enlarged section of one of
the packing rings is shown at (_a_). A front elevation of the
locking arrangement for the piston rod nut is shown at (_b_). A
sectional plan of one of the nuts for the junk ring bolts is shown
at (_c_).
First draw the vertical section of this piston, next draw a plan,
one-third of which is to show the piston complete, one-third to
show the junk ring removed, and the remaining third to be a
horizontal section through between the packing rings. The details
(_a_) and (_c_) need not be drawn separately. Scale 3 inches to a
foot.
_Proportions of Marine Engine Pistons._--Mr. Seaton, in his 'Manual of
Marine Engineering,' gives the following rules for designing marine
engine pistons:--
D = diameter of piston in inches.
_p_ = effective pressure in lbs. per square inch.
_x_ = D/50 × [sqrt (_p_)] + 1.
Thickness of front of piston near boss 0.2 × _x_.
" " " rim 0.17 × _x_.
" back of piston 0.18 × _x_.
" boss around rod 0.3 × _x_.
" flange inside packing ring 0.23 × _x_.
" " at edge 0.25 × _x_.
" junk ring at edge 0.23 × _x_.
" " inside packing ring. 0.21 × _x_.
" " at bolt-holes 0.35 × _x_.
" metal around piston edge 0.25 × _x_.
Breadth of packing ring 0.63 × _x_.
Depth of piston at centre 1.4 × _x_.
Lap of junk ring on piston 0.45 × _x_.
Space between piston body and packing ring 0.3 × _x_.
Diameter of junk-ring bolts 0.1 × _x_ + .25 inch.
Pitch of junk-ring bolts 10 diameters.
Number of webs in piston (D + 20)/12.
Thickness " 0.18 × _x_.
EXERCISE 50: _Design for Marine Engine Piston._--Calculate by
Seaton's rules the dimensions for a marine engine piston 40 inches
in diameter, and subjected to an effective pressure of 36 lbs. per
square inch. Then make the necessary working drawings for this
piston to a scale of, say, 3 inches to a foot.
_Note._--Take the dimensions got by calculation to the nearest
1-16th of an inch.
XV. STUFFING-BOXES.
[Illustration: FIG. 50.]
In fig. 50 is shown a gland and stuffing-box for the piston rod of a
vertical engine. A B is the piston rod, C D a portion of the cylinder
cover, and E F the _stuffing-box_. Fitting into the bottom of the
stuffing-box is a brass bush H. The space K around the rod A B is filled
with _packing_, of which there is a variety of kinds, the simplest
being greased hempen rope. The packing is compressed by screwing down
the cast-iron gland L M, which is lined with a brass bush N. In this
case the gland is screwed down by means of three stud-bolts P, which are
screwed into a flange cast on the stuffing-box. Surrounding the rod on
the top of the gland there is a recess R for holding the lubricant.
[Illustration: FIG. 51.]
[Illustration: FIG. 52.]
The object of the gland and stuffing-box is to allow the piston rod to
move backwards and forwards freely without any leakage of steam.
Fig. 51 shows a gland and stuffing-box for a horizontal rod. The
essential difference between this example and the last is in the mode of
lubrication. The gland flange has cast within it an oil-box which is
covered by a lid; this lid is kept shut or open by the action of a small
spring as shown. A piece of cotton wick (not shown in the figure) has
one end trailing in the oil in the oil-box, while the other is carried
over and passed down the hole A B. The wick acts as a siphon, and drops
the oil gradually on to the rod. In this example only two bolts are used
for screwing in the gland; and the flanges of the gland and stuffing-box
are not circular, but oval-shaped.
In the case of small rods the gland is made entirely of brass, and no
liner is then necessary. Fig. 52 shows a form of gland and stuffing-box
sometimes used for small rods. The stuffing-box is screwed externally,
and carries a nut A B which moves the gland.
EXERCISE 51: _Gland and Stuffing-box for a Vertical Rod._--Draw the
views shown in fig. 50 to the dimensions given. Scale 6 inches to a
foot.
EXERCISE 52: _Gland and Stuffing-box for a Horizontal Rod._--Fig.
51 shows a plan, half in section, and an elevation half of which is
a section through the gland flange. Draw these to a scale of 6
inches to a foot, using the dimensions marked in the figure.
EXERCISE 53: _Screwed Gland and Stuffing-box._--Draw, full size,
the views shown in fig. 52 to the given dimensions.
A more elaborate form of gland and stuffing-box is shown in fig. 53.
This is for a large marine engine with inverted cylinders, such as is
used on board large ocean steamers. The stuffing-box is cast separate
from the cylinder cover to which it is afterwards bolted. The lubricant
is first introduced to the oil-boxes marked A, from which it passes to
the recess B, where it comes in contact with the piston rod. To prevent
the lubricant from being wasted by running down the rod, the main gland
is provided with a shallow gland and stuffing-box which is filled with
soft cotton packing, which soaks up the lubricant.
The main gland is screwed up by means of six bolts, and to prevent the
gland from locking itself in the stuffing-box, it is necessary that the
nuts should be turned together. This is done in a simple and ingenious
manner. One-half of each nut is provided with teeth, and these gear with
a toothed wheel which has a rim only; this rim is held up by a ring C.
When one nut is turned, all the rest follow in the same direction.
[Illustration: FIG. 53.]
EXERCISE 54: _Gland and Stuffing-box for Piston Rod of Large
Inverted Cylinder Engine._--The lower view in fig. 53 is a half
plan looking upwards, and a half section of the gland looking
downwards. The upper view is a vertical section. Complete all these
views and add an elevation. Scale 3 inches to a foot.
_Note._--The large nuts, the wheel, the supporting ring, and small
gland are made of brass.
_Dimensions of Stuffing-boxes and Glands._
_d_ = diameter of rod. _t__{1} = thickness of
_d__{1} = diameter of box (inside). stuffing-box flange.
_l_ = length of stuffing-box _t__{2} = thickness of gland
bush. flange.
_l__{1} = length of packing space. _t__{3} = thickness of bushes in
_l__{2} = length of gland. box and gland.
_t_ = thickness of metal in _d__{2} = diameter of gland bolts.
stuffing-box. _n_ = number of bolts.
+----------------------------------------------------------+
| _d_ | _d__{1} | _l_ | _l__{1} | _l__{2} | _t_ | _t__{1} |
+-----+---------+-----+---------+---------+------+---------+
|1 | 1-3/4 | 3/4| 2 | 1-1/2 | 7/16| 1/2 |
|1-1/2| 2-1/2 |1-1/4| 2-5/8 | 2 | 9/16| 11/16 |
|2 | 3-1/2 |1-3/4| 3-1/4 | 2-1/2 | 11/16| 7/8 |
|2-1/2| 4-1/8 |2-1/4| 3-7/8 | 2-7/8 | 13/16| 1-1/16 |
|3 | 4-3/4 |2-3/4| 4-1/2 | 3-1/4 | 15/16| 1-1/4 |
|3-1/2| 5-1/4 | 3 | 5-1/8 | 3-5/8 |1 | 1-3/8 |
|4 | 5-7/8 |3-1/4| 5-3/4 | 4 |1 | 1-3/8 |
|4-1/2| 6-3/8 |3-1/2| 6-3/8 | 4-3/8 |1-1/16| 1-9/16 |
|5 | 7 |3-3/4| 7 | 4-5/8 |1-1/16| 1-9/16 |
|6 | 8 |4-1/4| 8-1/4 | 5 |1-1/8 | 1-11/16 |
+----------------------------------------------------------+
+-------------------------------------------------+
| _d_ | _t__{2} | _t__{3} | _d__{2} | _n_ |
+-----+-----------------+---------+---------+-----+
|1 | _t__{2}=_t_ | 3/16 | 7/16 | 2 |
|1-1/2| when gland | 1/4 | 5/8 | 2 |
|2 | flange is | 5/16 | 3/4 | 2 |
|2-1/2| made of cast | 5/16 | 7/8 | 2 |
|3 | iron and | 3/8 | 1 | 2 |
|3-1/2| _t__{2}=_t__{1} | 3/8 | 1 | 2 |
|4 | when gland | 7/16 | 1 | 2 |
|4-1/2| flange is | 7/16 | 7/8 | 4 |
|5 | made of | 7/16 | 1 | 4 |
|6 | brass. | 1/2 | 1-1/4 | 4 |
+-------------------------------------------------+
The proportions of glands and stuffing-boxes vary considerably but the
above table represents average practice.
EXERCISE 55:--Make the necessary working drawings for a gland and
stuffing-box for a locomotive engine piston rod 2-1/2 inches in
diameter, to the dimensions given in the table.
XVI. VALVES.
Professor Unwin divides valves, according to their construction into
three classes as follows:--(1) flap valves, which bond or turn upon a
hinge; (2) lift valves, which rise perpendicularly to the seat; (3)
sliding valves, which move parallel to the seat.
Examples of flap valves are shown in figs. 54 and 55; two forms of lift
valves are shown in figs. 56 and 57, and in figs. 58 and 59 are shown
two forms of slide valve. The slide valve shown in fig. 58 moves in a
straight line, while that shown in fig. 59 (called a cock) moves in
circle.
_India-rubber Valves._--In india-rubber valves there is a grating
covered by a piece of india-rubber, which may be rectangular, but is
generally circular, and which is held down along one edge if
rectangular, or at the centre if circular. Water or other fluid can pass
freely upwards through the grating, but when it attempts to return the
elasticity of the india-rubber, and the pressure of the water upon it,
cause it to lie close on the grating, and thus prevent the return of the
water. The india-rubber is prevented from rising too high by a
perforated guard. In fig. 54 is shown an example of an india-rubber disc
valve. A is the grating, B the india-rubber, C the guard secured to the
grating or seat by the stud D and nut E. The grating is held in position
by bolts and nuts F. The grating and guard are generally of brass.
India-rubber disc valves are also shown on the air-pump bucket, fig. 47.
EXERCISE 56: _India-rubber Disc Valve._--Fig. 54 shows a vertical
section and a plan of an india-rubber disc valve. In the plan
one-half of the guard and india-rubber are supposed to be removed
so as to show the grating or seat. Draw these views, and also an
elevation. A detail drawing of the central stud is shown in fig.
16, page 18. In fig. 54 the elevation of the guard is drawn as it
is usually drawn in practice, but if the student has a sufficient
knowledge of descriptive geometry he should draw the elevation
completely showing the perforations. Scale 6 inches to a foot.
[Illustration: FIG. 54.]
[Illustration: FIG. 55.]
_Kinghorn's Metallic Valve._--The action of this valve is the same as
that of an india-rubber valve, but a thin sheet of metal (phosphor
bronze) takes the place of the india-rubber.
This valve is now largely used in the pumps of marine engines, and is
shown in fig. 55 as applied to an air-pump bucket. Three valves like the
one shown are arranged round the bucket.
EXERCISE 57: _Kinghorn's Metallic Valve._--Fig. 55 shows an
elevation and plan of one form of this valve. In the plan one-half
of the guard and metal sheet are supposed to be removed, so as to
show the grating, which in this case is part of an air-pump bucket.
Draw the views shown, and also a vertical section of the guard
through the centres of the bolts. All the parts are of brass except
the valve proper, which is of phosphor bronze. Scale 6 inches to a
foot.
_Conical Disc Valves._--A very common form of valve is that shown in
figs. 56 and 57. This form of valve consists of a disc, the edge of
which (called the face) is conical. The conical edge of this disc fits
accurately on a corresponding seat. The angle which the valve face makes
with its axis is generally 45°. If the disc is raised, either by the
action of the fluid as in the india-rubber valve, or by other means, an
opening is formed around the disc through which the fluid can pass. The
valve is guided in rising and falling either by three feathers
underneath it, as in fig. 56, or by a central spindle which moves freely
through a hole in the centre of a bridge which stretches across the
seat, as in fig. 57. The lift of the valve is limited by a stop above
it, which forms part of the casing containing the valve. The lift should
in no case exceed one-fourth of the diameter of the valve, and it is
generally much less than this. The guiding feathers (fig. 56) are
notched immediately under the disc for the purpose of making available
the full circumferential opening of the valve for the passage of the
fluid. These notches also prevent the feathers from interfering with the
turning or scraping of the valve face.
Conical disc valves and their seats are nearly always made of brass.
EXERCISE 58: _Conical Disc Valves._--Draw, half size, the plans and
elevations shown in figs. 56 and 57. In fig. 57 the valve is shown
open in the elevation, and in the plan it is removed altogether in
order to show the seat with its guide bridge.
[Illustration: Plan of Valve. FIG. 56.]
[Illustration: Plan of Seat. FIG. 57.]
_Simple Slide Valve._--The form of valve shown in fig. 58, often called
the _locomotive slide valve_, is very largely used in all classes of
steam-engines for distributing the steam in the steam cylinders. The
valve is shown separately at (_d_), (_e_), and (_f_), while at (_a_),
(_b_), and (_c_) is shown its connection with the steam cylinder.
It will be observed that the valve itself is in the shape of a box with
one side open, the edges of the open side being flanged. When the valve
is in its middle position, as shown at (_a_), two of these flanged edges
completely cover two rectangular openings S_{1} and S_{2}, called _steam
ports_, while the hollow part of the valve is opposite to a third port
E, called the _exhaust port_. As shown at (_a_) the piston P would be
moving upwards and the valve downwards. By the time the piston has
reached the top of its stroke the valve will have moved so far down as
to partly uncover the steam port S_{1}, and admit steam from the valve
casing C through S_{1} and the passage P_{1} to the top of the piston.
The pressure of this steam on the top of the piston will force the
latter down. While the above action has been going on, the port S_{2}
will have become uncovered, and the hollow part of the valve will be
opposite both the steam port S_{2} and the exhaust port E, so that the
steam from the under side of the piston, and which forced the piston up,
can now escape by the passage P_{2}, the steam port S_{2}, and the
exhaust port E to the exhaust outlet O, and thence into the atmosphere,
if it is a non-condensing engine, or into the condenser if it is a
condensing engine, or into another cylinder if it is a compound engine.
After the piston has performed, a certain part of its downward stroke,
the valve, which has been moving downwards, will commence to move
upwards, and when it has reached a certain point it will cover the port
S_{1}, and shut off the supply of steam to the top of the piston. It is
generally arranged that the steam shall be cut off before the piston
reaches the end of the stroke. When the piston reaches the bottom of its
stroke the valve has moved far enough up to uncover the port S_{2} and
admit steam to the bottom of the piston, and to uncover the port S_{1}
and allow the steam to escape from the top of the piston through the
passage P_{1}, the port S_{1}, the port E, and outlet O. In this way the
piston is moved up and down in the cylinder.
The valve is attached to a valve spindle S by nuts as shown, the hole in
the valve through which the spindle passes being oval-shaped to permit
of the valve adjusting itself so as to always press on its seat.
When the valve is in its middle position it generally more than covers
the steam ports. The amount which the valve projects over the steam port
on the outside, the valve being in its middle position, is called the
_outside lap_ of the valve, and the amount which it projects on the
inside is called the _inside lap_. When the term lap is used without any
qualification, outside lap is to be understood. In fig. 58 it will be
seen that the valve has no inside lap, and that the outside lap is
three-eighths of an inch. The inside lap is generally small compared
with the outside lap.
[Illustration: FIG. 58.]
When the piston is at the beginning of its stroke the steam port is
generally open by a small amount called the _lead_ of the valve.
The reciprocating motion of the slide valve is nearly always derived
from an eccentric fixed on the crank-shaft of the engine. Slide valves
are generally made of brass, bronze, or cast iron.
EXERCISE 59: _Simple Slide Valve._--At (_d_), fig. 58, is shown a
sectional elevation of a simple slide valve for a steam-engine, the
section being taken through the centre line of the valve spindle,
while at (_e_) is shown a cross section and elevation, and at (_f_)
a plan of the same. Draw all these views full size, and also a
sectional elevation at A B. The valve is made of brass, and the
valve spindle and nuts of wrought iron.
EXERCISE 60: _Slide Valve Casing, &c., for Steam-engine._--Draw,
half size, the views shown at (_a_), (_b_), and (_c_), fig. 58;
also a sectional plan at L M. (_b_) is an elevation of the valve
casing with the cover and the valve removed. (_a_) is a sectional
elevation, the section being taken through the axes of the steam
cylinder and valve spindle. (_c_) is a sectional plan, the section
being a horizontal one through the centre of the exhaust port. The
inlet and outlet for the steam are clearly shown in the sectional
plan: in the sectional elevation their positions are shown by
dotted circles.
The stroke of the piston is in this case 12 inches, so that from
the dimensions given at (_a_) it must come within a quarter of an
inch of each end of the cylinder; this is called the _cylinder
clearance_.
The piston has three Ramsbottom rings, a quarter of an inch wide
and a quarter of an inch apart.
The steam cylinder and valve casing are made of cast iron.
_Cocks._--A cock consists of a slightly conical plug which fits into a
corresponding casing cast on a pipe. Through the plug is a hole which
may be made by turning the plug to form a continuation of the hole in
the pipe, and thus allow the fluid to pass, or it may be turned round so
that the solid part of the plug lies across the hole in the pipe, and
thus prevent the fluid from passing. As the student will be quite
familiar with the common water cock or tap such as is used in
dwelling-houses we need not illustrate it here.
[Illustration: FIG. 59.]
Fig. 59 shows a cock of considerable size, which may be used for water
or steam under high pressure. The plug in this example is hollow, and is
prevented from coming out by a cover which is secured to the casing by
four stud bolts. An annular ridge of rectangular section projecting from
the under side of the cover, and fitting into a corresponding recess on
the top of the casing, serves to ensure that the cover and plug are
concentric, and prevents leakage. Leakage at the neck of the plug is
prevented by a gland and stuffing-box. The top end of the plug is made
square to receive a handle for turning it. The size of a cock is taken
from the bore of the pipe in which it is placed; thus fig. 59 shows a
2-1/4-inch cock.
EXERCISE 61: 2-1/4-_inch Steam or Water Cock._--First draw the
views of this cock shown in fig. 59, then draw a half end elevation
and half cross section through the centre of the plug. Scale 6
inches to a foot.
Instead of drawing the parts of the pipe on the two sides of the
plug in the same straight line as in fig. 59, one may be shown
proceeding from the bottom of the casing, so that the fluid will
have to pass through the bottom of the plug and through one side.
This is a common arrangement.
All the parts of the valve and casing in this example are made of
brass.
XVII. MATERIALS USED IN MACHINE CONSTRUCTION.
_Cast Iron._--The essential constituents of cast iron are iron and
carbon, the latter forming from 2 to 5 per cent. of the total weight.
Cast iron, however, usually contains varying small amounts of silicon,
sulphur, phosphorus, and manganese.
In cast iron the carbon may exist partly in the free state and partly in
chemical combination with the iron.
In _white cast iron_ the whole of the carbon is in chemical combination
with the iron, while in _grey cast iron_ the carbon is principally in
the free state, that is, simply mixed mechanically with the iron. It is
the free carbon which gives the grey iron its dark appearance. A mixture
of the white and grey varieties of cast iron when melted produces
_mottled cast iron_. The greater the amount of carbon chemically
combined with the iron, the whiter, harder, and more brittle does it
become.
The white cast iron is stronger than the grey, but being more brittle it
is not so suitable for resisting suddenly applied loads. White iron
melts at a lower temperature than grey iron, but after melting it does
not flow so well, or is not so liquid as the grey iron. White iron
contracts while grey iron expands on solidifying. The grey iron,
therefore, makes finer castings than the white. Castings after
solidifying contract in cooling about 1/8 of an inch per foot. Castings
possessing various degrees of strength and hardness are produced by
melting mixtures of various proportions of white and grey cast irons.
White cast iron has a higher specific gravity than grey cast iron.
Cast iron gives little or no warning before breaking. The thickness of
the metal throughout a casting in cast iron should be as uniform as
possible, so that it may cool and therefore contract uniformly
throughout; otherwise some parts may be in a state of initial strain
after the casting has cooled, and will therefore be easier to fracture.
Re-entrant angles should be avoided; such should be rounded out with
fillets.
The presence of phosphorus in cast iron makes it more fusible, and also
more brittle. The presence of sulphur diminishes the strength
considerably.
The grey varieties of cast iron are called _foundry irons_ or _foundry
pigs_, while the white varieties are called _forge irons_ or _forge
pigs_, from the fact that they are used for conversion into wrought
iron.
Amongst iron manufacturers the different varieties of cast iron are
designated by the numbers 1, 2, 3, &c., the lowest number being applied
to the greyest variety.
_Chilled Castings._--When grey cast iron is melted a portion of the free
carbon combines chemically with the iron; this, however, separates out
again if the iron is allowed to cool slowly; but if it is suddenly
cooled a greater amount of the carbon remains in chemical combination,
and a whiter and harder iron is produced. Advantage is taken of this in
making _chilled castings_. In this process the whole or a part of the
mould is lined with cast iron, which, being a comparatively good
conductor of heat, chills a portion of the melted metal next to it,
changing it into a hard white iron to a depth varying from 1/8 to 1/2 an
inch. To protect the cast-iron lining of the mould from the molten metal
it is painted with loam.
_Malleable Cast Iron._--This is prepared by imbedding a casting in
powdered red hematite (an oxide of iron), and keeping it at a bright red
heat for a length of time varying from several hours to several days
according to the size of the casting. By this process a portion of the
carbon in the casting is removed, and the strength and toughness of the
latter become more like the strength and toughness of wrought or
malleable iron.
_Wrought or Malleable Iron._--This is nearly pure iron, and is made from
cast iron by the puddling process, which consists chiefly of raising the
cast iron to a high temperature in a reverberatory furnace in the
presence of air, which unites with the carbon and passes off as gas. In
other words the carbon is burned out. The iron is removed from the
puddling furnace in soft spongy masses called _blooms_, which are
subjected to a process of squeezing or hammering called _shingling_.
These shingled blooms still contain enough heat to enable them to be
rolled into rough _puddled bars_. These puddled bars are of very
inferior quality, having less than half the strength of good wrought
iron. The puddled bars are cut into pieces which are piled together,
reheated, and again rolled into bars, which are called _merchant bars_.
This process of piling, reheating, and re-rolling may be repeated
several times, depending on the quality of iron required. Up to a
certain point the quality of the iron is improved by reheating and
rolling or hammering, but beyond that a repetition of the process
diminishes the strength of the iron.
The process of piling and rolling gives wrought iron a fibrous
structure. When subjected to vibrations for a long time, the structure
becomes crystalline and the iron brittle. The crystalline structure
induced in this way may be removed by the process of _annealing_, which
consists in heating the iron in a furnace, and then allowing it to cool
slowly.
_Forging and Welding._--The process of pressing or hammering wrought
iron when at a red or white heat into any desired shape is called
_forging_. If at a white heat two pieces of wrought iron be brought
together, their surfaces being clean, they may be pressed or hammered
together, so as to form one piece. This is called _welding_, and is a
very valuable property of wrought iron.
_Steel._--This is a compound of iron with a small per-centage of carbon,
and is made either by adding carbon to wrought iron, or by removing some
of the carbon from cast iron.
In the _cementation_ process, bars of wrought iron are imbedded in
powdered charcoal in a fireclay trough, and kept at a high temperature
in a furnace for several days. The iron combines with a portion of the
carbon to form _blister steel_, so named because of the blisters which
are found on the surface of the bars when they are removed from the
furnace.
The bars of blister steel are broken into pieces about 18 inches long,
and tied together in bundles by strong steel wire. These bundles are
raised to a welding heat in a furnace, and then hammered or rolled into
bars of _shear steel_.
To form _cast steel_ the bars of blister steel are broken into pieces
and melted into crucibles.
In the _Siemens-Martin_ process for making steel, cast and wrought iron
are melted together on the hearth of a regenerative gas-furnace.
_Bessemer steel_ is made by pouring melted cast iron into a vessel
called a converter, through which a blast of air is then urged. By this
means the carbon is burned out, and comparatively pure iron remains. To
this is added a certain quantity of 'spiegeleisen,' which is a compound
of iron, carbon, and manganese.
_Hardening and Tempering of Steel._--Steel, if heated to redness and
cooled suddenly, as by immersion in water, is hardened. The degree of
hardness produced varies with the rate of cooling; the more rapidly the
heated steel is cooled, the harder does it become. Hardened steel is
softened by the process of _annealing_, which consists in heating the
hardened steel to redness, and then allowing it to cool slowly. Hardened
steel is _tempered_, or has its degree of hardness lowered, by being
heated to a temperature considerably below that of a red heat, and then
cooling suddenly. The higher the temperature the hardened steel is
raised to, the lower does its 'temper' become.
_Case-hardening._--This is the name given to the process by which the
surfaces of articles made of wrought iron are converted into steel, and
consists in heating the articles in contact with substances rich in
carbon, such as bone-dust, horn shavings, or yellow prussiate of potash.
This process is generally applied to the articles after they are
completely finished by the machine tools or by hand. The coating of
steel produced on the article by this process is hardened by cooling the
article suddenly in water.
_Copper._--This metal has a reddish brown colour, and when pure is very
malleable and ductile, either when cold or hot, so that it may be rolled
or hammered into thin plates, or drawn into wire. Slight traces of
impurities cause brittleness, although from 2 to 4 per cent. of
phosphorus increases its tenacity and fluidity. Copper is a good
conductor of heat and of electricity. Copper is largely used for making
alloys.
_Alloys._--_Brass_ contains two parts by weight of copper to one of
zinc. _Muntz metal_ consists of three parts of copper to two of zinc.
Alloys consisting of copper and tin are called _bronze_ or _gun-metal_.
Bronze is harder the greater the proportion of tin which it contains;
five parts of copper to one of tin produce a very hard bronze, and ten
of copper to one of tin is the composition of a soft bronze. _Phosphor
bronze_ contains copper and tin with a little phosphorus; it has this
advantage over ordinary bronze, that it may be remelted without
deteriorating in quality. This alloy also has the advantage that it may
be made to possess great strength accompanied with hardness, or less
strength with a high degree of toughness.
_Wood._--In the early days of machines wood was largely used in their
construction, but it is now used to a very limited extent in that
direction. _Beech_ and _hornbeam_ are used for the cogs of mortise
wheels. _Yellow pine_ is much used by pattern-makers. _Box_, a heavy,
hard, yellow-coloured wood, is used for the sheaves of pulley blocks,
and sometimes for bearings in machines. _Lignum-vitæ_ is a very hard
dark-coloured wood, and remarkable for its high specific gravity, being
1-1/3 times the weight of the same volume of water. This wood is much
used for bearings of machines which are under water.
XVIII. MISCELLANEOUS EXERCISES.
The illustrations in this chapter are in most cases not drawn to scale;
they are also in some parts incomplete, and in others some of the lines
are purposely drawn wrong. The student must keep to the dimensions
marked on the drawings, and where no sizes are given he must use his own
judgment in proportioning the parts. All errors must be corrected, and
any details required, but not shown completely in the illustrations,
must be filled in.
EXERCISE 62: _Single Riveted Butt Joint with Tee-iron Cover
Strap._--Two views, one a side elevation and the other a sectional
elevation, of a riveted joint are shown in fig. 60. Draw these
views, and also a plan projected from one of them. Show the rivets
completely in all the views. Scale 4 inches to a foot.
[Illustration: FIG. 60.]
[Illustration: FIG. 61.]
EXERCISE 63: _Girder Stay for Steam Boiler._--The flat crown of the
fire-box of locomotive and marine boilers is generally supported or
stayed by means of girder stays, an example of which is shown in
fig. 61. A B is the side elevation of a portion of one of these
girders. Each girder is supported at its ends by the plates forming
the vertical sides of the fire-box. The flat crown is bolted to the
girders as shown. Observe that the girders are in contact with the
crown only in the neighbourhood of the bolts. Consider carefully
this part of the design, and then answer the following questions:
(1) What objections are there to supporting the girders at the ends
only without the contact pieces at the bolts? (2) What objections
are there to having the girders in contact with the crown plate of
the fire-box throughout their whole length?
Draw the views shown in fig. 61, and from the right-hand one
project a plan. Scale 4 inches to a foot.
[Illustration: FIG. 62.]
EXERCISE 64: _End of Bar Stay for Steam Boiler._--On page 12 one
form of stay for supporting the flat end of a steam boiler is
described. Another form of stay for the same purpose is shown in
fig. 62. A B is a portion of the end of a steam boiler. C D is one
end of a bar which extends from one end of the boiler to the
other. The ends of this bar are screwed, and when the bar is of
wrought iron the screwed parts are generally larger in diameter
than the rest of the bar. When made of steel the bar is generally
of uniform diameter throughout. In the case of wrought-iron bar
stays the enlarged ends are welded on to the smaller parts.
Welding is not so reliable with steel as with wrought iron. Write
out answers to the following questions: (1) What is the advantage
of having the screwed part of the bar larger in diameter than the
rest? (2) Why are steel bar stays not generally enlarged at their
screwed ends?
Draw the views shown in fig. 62, and project from one of them a
third view. Scale 4 inches to a foot.
EXERCISE 65: _Knuckle Joint._--Draw the plan and elevation of this
joint shown in fig. 63, and also draw an end elevation looking in
the direction of the arrow. The parts at A and B are octagonal in
cross section. Scale 4 inches to a foot.
[Illustration: FIG. 63.]
EXERCISE 66: _Locomotive Coupling Rod Ends._--A form of knuckle
joint used on locomotive coupling rods is shown in fig. 64.
In this case two rods meet and work on the same pin, as shown at
(a) fig. 64. Draw, in addition to the views shown in fig. 64, a
plan and a vertical section through the axis of the pin. Scale 6
inches to a foot.
Would it be practicable to replace the two rods A B and B C by a
single rod working on the crank pins at A, B, and C? Give reasons
for your answer.
[Illustration: FIG. 64.]
EXERCISE 67: _Bell Crank Lever._--Draw the plan and elevation of
the lever shown in fig. 65. Scale 6 inches to a foot.
[Illustration: FIG. 65.]
EXERCISE 68: _Back Stay for Lathe._--Draw a plan and two elevations
of the stay shown in fig. 66. Make all necessary corrections and
show all the details in each view. Scale full size.
[Illustration: FIG. 66.]
[Illustration: FIG. 67.]
EXERCISE 69: _Conical Disc Valve and Casing._--Draw, half size, the
views shown in fig. 67 of the conical disc valve and casing, and
also add an elevation looking in the direction of the arrow.
EXERCISE 70: _Connecting Rod End._--The student should carefully
compare this connecting rod end (fig. 68) with those illustrated on
pages 50 and 52. The lower part of fig. 68 is a half plan and half
horizontal section, and the upper part is a half side elevation and
half vertical section. Draw these views and also an end elevation.
Scale 6 inches to a foot.
[Illustration: FIG. 68.]
[Illustration: FIG. 69.]
[Illustration: FIG. 70.]
[Illustration: FIG. 71.]
[Illustration: FIG. 72.]
[Illustration: FIG. 73.]
[Illustration: FIG. 74.]
EXERCISE 71: _Engine Cross-head._--The cross-head shown in fig. 69
is for an inverted cylinder marine engine. A is the piston rod, and
B B are pins, forged in one piece with C, to which the forked end
of the connecting rod is attached. Draw the upper view with the
central part in section as shown. Make the right-hand half of the
lower view a plan without any section, and make the left-hand half
a horizontal section through the axis of the pins B B. Scale 4
inches to a foot.
EXERCISE 72: _Ratchet Lever._--The lever shown in fig. 70 is used
for turning the horizontal screw of a traversing screw jack. Draw
the two views shown, and from one of them project a plan. Scale
full size.
EXERCISE 73: _Steam Whistle._--Draw, full size, the elevation and
section of the steam whistle shown in fig. 71. Draw also horizontal
sections at A B, C D, and E F.
[Illustration: FIG. 75.]
EXERCISE 74: _Screw Coupling for Railway Carriages._--Draw the
three views of the screw coupling shown in fig. 72. Scale 6 inches
to a foot.
If the link A is fixed, through what distance will the link B move
for two turns of the lever?
[Illustration: FIG. 76.]
EXERCISE 75: _Loose Headstock for a 6-inch Lathe._--Two views of
this headstock are shown in fig. 73. On one of these views a few of
the chief dimensions are marked. The details, fully dimensioned,
are shown separately in figs. 74, 75, and 76.
Explain clearly how the centre is moved backwards and forwards, and
also how the spindle containing it is locked when it is not
required to move.
Draw, half-size, the views shown in fig. 73, and from the
left-hand view project a plan. Draw also the detail of the locking
arrangement shown in fig. 74.
APPENDIX A.
_SCIENCE AND ART DEPARTMENT, SOUTH KENSINGTON._
SYLLABUS.
SUBJECT II.--MACHINE CONSTRUCTION AND DRAWING.
It is assumed that the student has already learnt to draw to scale, and
that he can draw two or more views of the same object in simple or
orthographic projection. To pass in machine construction and drawing, he
must be able to apply this knowledge to the representation of machinery.
He must be acquainted with the form and purpose of the simpler parts of
which machines are built up and must have had some practice in drawing
them. To test his knowledge, rough dimensioned sketches, more or less
incomplete, of simple machine details will be given him, and he will be
required to produce a complete drawing in pencil to a given scale. Two
or more views of at least one subject will be required, and these must
be so drawn as to be properly projected one from the other, _in order to
show that the student appreciates that he is producing a representation
of a solid piece of machinery, and not merely copying a sketch. No
credit will be given unless some knowledge of projection is shown._ The
centre lines of the drawings should be shown, and parts cut by planes of
section should be indicated by diagonal shading. Bolts and other
fastenings should be carefully shown where required. Any indication that
a candidate has merely copied the sketches given, without understanding
the part represented, will invalidate his examination.
FIRST STAGE OR ELEMENTARY COURSE.
In the elementary stage, a knowledge is required of the simple parts
only of _machines in common use_. _Some_ of these are enumerated in the
following list. The student should be practised in drawing them till he
recognises their forms, and the object of the arrangement should be
explained to him. He should also know the simple technical terms used in
describing them.
A few very simple questions relating to the arrangement, proportions,
and strength of the simplest machine details will be set in the
examination paper.
In drawing the examples set to test a student's knowledge and skill in
machine drawing, it must be remembered that only a limited time is
available. It is only possible to set an example to be drawn in pencil,
and the points which will receive attention are (1) accuracy of scale
and projection; (2) power of reading a drawing, shown by the ability to
transfer portions of the mechanism and dimensions from one view to
another; (3) knowledge of machines, as shown by the ability to fill in
small details, such as nuts, keys, etc., omitted in the sketches given.
Bearing in mind the limited time available, the student should try to
make his outline clear and decisive and complete. But the diagonal lines
necessary for sectional parts may be done rapidly, though neatly, by
freehand if necessary.
_Riveted Joints._--Forms of rivets and arrangement of rivets in lap and
butt joints with single and double riveting. Junction of plates by angle
and T-irons.
_Bolts, Studs, and Set Screws._--Forms of these fastenings. Forms and
proportions of nuts and bolt-heads. Arrangement of flanges for bolting.
_Pins, Keys, and Cotters._--Form of ordinary knuckle joint. Use of split
pins. Connection of parts by a key. Connection of parts by a cotter. Gib
and cotter.
_Pipes and Cylinders._--Forms of ordinary pipes and cylinders and their
flanges and covers.
_Shafting._--Forms of shafts and axles and of journals and pivots. Use
of collars and bosses. Half-lap coupling. Box coupling. Flange coupling.
_Pedestals and Plummer Blocks._--Simplest forms of pedestals and hangers
for shafts. Form and arrangement of brass steps. Arrangements for
fixing pedestals and for neutralising the effects of wear.
_Toothed Gearing._--Forms of ordinary spur and bevil wheels. Meaning of
the terms pitch, breadth of face, thickness of tooth, pitch line, rim,
nave, arm. Mode of drawing bevil wheels in section.
_Belt Pulleys._--Forms of belt pulleys for flat and round belts. Stepped
speed cones. Drawing of pulleys with curved arms.
_Cranks and Levers._--Forms of ordinary cast-iron and wrought-iron
cranks and levers. Modes of fixing crank pin. Modes of fixing crank
shaft. Double cranks. Form of eccentrics.
_Links._--Most simple forms of connecting rod ends, open or closed. Use
of steps in connecting rods. Use of cotters to tighten the steps.
_Pistons._--Simple forms of piston. Use of piston packing. Modes of
attaching piston rod.
_Stuffing-Boxes._--Simple form of stuffing-box and gland. Use of
packing. Mode of tightening gland.
_Valves._--Simple conical of puppet valve. Simple slide valve. Cock or
conical sliding valve.
APPENDIX B.
_EXAMINATION PAPERS SET BY THE SCIENCE AND ART DEPARTMENT._
SUBJECT II.--MACHINE CONSTRUCTION AND DRAWING.
_Examiners_, PROF. T. A. HEARSON, M.Inst.C.E., and J. HARRISON, ESQ.,
M.Inst.M.E.
GENERAL INSTRUCTIONS.
_If the rules are not attended to, the paper will be cancelled._
You may take the Elementary, or the Advanced, or the Honours paper, but
you must confine yourself to one of them.
Put the number of the question before your answer.
You are expected to prove your knowledge of machinery as well as your
power of drawing neatly to scale. You are therefore to supply details
omitted in the sketches, to fill in parts left incomplete, and to
indicate, by diagonal lines, parts cut by planes of section.
No credit will be given unless some knowledge of projection is shown, so
that at least two views of one of the examples will be required properly
projected one from the other. The centre lines should be clearly drawn.
The figured dimensions need not be inserted.
Your answers should be clearly and cleanly drawn in pencil. No extra
marks will be allowed for inking in.
All figures must be drawn on the single sheet of paper supplied, for no
second sheet will be allowed.
The value attached to each question is shown in brackets after the
question. But a full and correct answer to an easy question will in all
cases secure a larger number of marks than an incomplete or inexact
answer to a more difficult one.
Your name is not given to the Examiner, and you are forbidden to write
to him about your answers.
You are to confine your answers _strictly_ to the questions proposed.
A single accent (') signifies _feet_; a double accent (") _inches_.
_The examination in this subject lasts for four hours._
* * * * *
First Stage or Elementary Examination. 1885.
INSTRUCTIONS.
Read the General Instructions above.
Answer briefly any three, but not more than three, of the following
questions, and draw two, but not more than two, of the examples.
_Questions._
(_a._) Show two methods by which a cotter may be prevented from
slacking back. (6.)
(_b._) Sketch the brasses for a bearing, and show how they are
prevented from turning in the pedestal. (6.)
(_c._) Explain the object of the construction of the connecting rod
end shown in fig. 78. Describe how the adjustment must be made and
how it is locked. (10.)
(_d._) Show the form of the Whitworth screw thread by drawing to
scale a part section of two or three threads taking a pitch of
1-1/2 inches. Figure the dimensions on the sketch. How many threads
to the inch are used on an inch bolt? (10.)
(_e._) Make a sketch showing how the adjustment is made in the
sliding parts of machine tools: as, for example, in the slide rest
of a lathe. (10.)
(_f._) Describe with sketches two methods by which the joints are
made in connecting lengths of cast-iron pipes. (6.)
_Examples to be drawn._
1. Jaw for four-screw dog chuck for 5" lathe. Draw the two views as
shown (fig. 77). Scale full size.
(Note.--The other three jaws of the chuck are not to be drawn.)
(35.)
2. Connecting rod end. Draw the two views as shown, partly in
section (fig. 78). Draw full size. (35.)
3. Hooke's coupling. Draw the three views shown (fig. 79), adding
any omitted lines where the views are incomplete. Draw to scale of
1/4 full size. (35.)
[Illustration: FIGS. 77 AND 78.]
[Illustration: FIG. 79.]
* * * * *
First Stage or Elementary Examination. 1886.
INSTRUCTIONS.
Read the General Instructions (page 102).
Answer briefly any three, but not more than three, of the following
questions, and draw two, but not more than two, of the examples.
_Questions._
(_a._) Give sketches showing how the cutting tool of a lathe or
other machine is secured in place. (6.)
(_b._) Make a sketch of a stud, describe how it is screwed into
place, and state some circumstances under which it is used in
preference to a bolt. (6.)
(_c._) Give sketches showing one method of attaching the valve rod
to an ordinary slide valve. (6.)
(_d._) Sketch a connecting rod end, with strap, gib, and cotter.
Explain the use of the gib. (10.)
(_e._) Explain the use of the quadrant for change wheels for a
screw-cutting lathe shown in Example 1, fig. 80, by making a
sketch showing it in place on a lathe with wheels in gear. (10.)
(_f._) Sketch one form of hanger suitable for supporting mill-shafting.
(10.)
_Examples to be drawn._
1. Quadrant for change wheels for screw-cutting lathe. Draw the two
views shown (fig. 80). Scale half-size. (35.)
2. Crank-shaft. Draw the two views as shown, partly in section (fig
81). Scale 1/8 full size. (35.)
3. Ball bearing for tricycle. Draw the two views as shown, partly
in section (fig. 82). Draw full size. (35.)
[Illustration: FIGS. 80 AND 81.]
[Illustration: FIG. 82.]
* * * * *
First Stage or Elementary Examination. 1887.
INSTRUCTIONS.
Read the General Instructions (page 102).
Answer briefly any three, but not more than three, of the following
questions, and draw two, but not more than two, of the examples.
_Questions._
(_a._) Explain how the piston rings in Example 1, fig. 84, are made
so that the piston may work steam-tight in the cylinder. How are
these rings got into place? (8.)
(_b._) Give two views of a double riveted lap joint for boiler-plates.
(8.)
(_c._) Show by sketches how a wheel is fixed on a shaft by means of
a sunk key. Explain how the key may be withdrawn when it cannot be
driven from the point end. (8.)
(_d._) Give sketches showing the construction of a conical metal
lift or puppet valve and seating. (10.)
(_e._) With the aid of sketches explain how a piston rod is made to
work steam-tight through the end of the cylinder. (10.)
(_f._) Explain how the slotting machine ram of Example 8, fig. 85,
may be made to move up and down when at work. How is the length of
the stroke altered, and what is the object of the slotway in the
upper part of the ram? (10.)
_Examples to be drawn._
1. Piston for steam-engine. Draw and complete the two views shown
(fig. 84), the top half of the left-hand view to be in section.
Scale 1/2 size. (30.)
2. Plan and sectional elevation of a footstep bearing for an
upright shaft (fig. 83). Draw and complete these views. Scale
1/4 size. (35.)
3. Ram of slotting machine. Draw and complete the two elevations
shown (fig. 85). The tool-holders must be drawn in their proper
positions in the ram, and not separate as in the diagram. Scale
1/4 size. (35.)
[Illustration: FIGS. 83 AND 84.]
[Illustration: FIG. 85.]
* * * * *
First Stage or Elementary Examination. 1888.
INSTRUCTIONS.
Read the General Instructions on p. 102.
Answer briefly any three, but not more than three, of the following
questions, and draw two, but not more than two, of the examples.
_Questions._
(_a._) Give sketches showing how the separate lengths of a line of
shafting may be connected together. (8.)
(_b._) What is the object of using chipping or facing strips in
fitting up machine parts? Give one or two examples. (8.)
(_c._) Give sketches showing how you would grip and drive a round
iron bar for the purpose of turning it between the centres of a
lathe. (10.)
(_d._) Explain the action of the governor shown in Example 1
(fig. 86). (10.)
(_e._) Describe in detail how the mud-hole door in Example 2
(fig. 88) is removed for the purpose of cleaning the boiler and
how it is replaced and the joint made steam-tight. (10.)
(_f._) Describe how the parts of the spur wheel in Example 3
(fig. 87) are put together, and explain why the wheel is made
in segments. (10.)
_Examples to be drawn._
1. Loaded governor for small gas engine. Draw and complete the two
views, partly in section as shown (fig. 86). Scale full size. (35.)
2. Mud-hole mouth-piece for Lancashire boiler. Draw and complete
the two views shown (fig. 88). Scale 3/8ths. (35.)
3. Point for segments of large spur wheel. Draw and complete the
views shown (fig. 87). Scale 3/16ths.
_Note._--As the radius of the wheel is too large for your
instruments, the circumference at the joint may be set out
straight, as in a rack. (35.)
[Illustration: FIGS. 86 AND 87.]
[Illustration: FIG. 88.]
INDEX
Air-pump bucket, 58
Alloys, 80
Angle irons, 12
Annealing, 79, 80
Axles, 24
Back stay for lathe, 86
Bar stay, 83
Bearings for shafts, 30
Beech-wood, 81
Bell crank lever, 86
Bessemer steel, 79
Bevil wheels, 43
Blister steel, 79
Blooms, 78
Bolt-heads, proportions of, 18
Bolts, forms of, 17
Border lines, 4
Box couplings, 25
-- end, connecting rod, 51
Box-wood, 81
Brackets, 33
Brake shaft carrier, 30
Brass, 80
Brasses, 30
Bucket, 58
Built-up cranks, 46
Bush, 30, 49, 51, 54, 56, 63
Butt joints, 10, 11
-- strap, 10
Buttress screw thread, 15
Case-hardening, 80
Cast iron, 76
Cast iron flange coupling, 28, 29
-- steel, 79
Caulking, 8
Cementation process, 79
Centre lines, 2, 4
Chilled castings, 78
Circulating pump piston, 58
Clearance, cylinder, 74
-- of cotter, 49
Cocks, 74
Cogs, 41
-- wood for, 81
Collared stud, 18
Collars, 24
Colouring, 3
Colours for different materials, 3
Compasses, 1
Cone keys, 23, 38
Conical disc valve, 70, 71, 89
-- head, 7
Connecting rod, locomotive, 51
-- -- marine, 51
-- rods, 49, 89
Construction for rivet heads, 7
Contraction of castings, 77
Copper, 80
Cotters, 48, 49
Countersunk head, 7, 18
Coupling rod ends, 55, 84
-- rods, 54
-- screw, 96
Couplings, shaft, 25
Cover plate, 10
Cranked axle, 45
Cranks, 43
-- built-up, 46
Cross-head pin, 51
Cross-heads, 56, 89
Cross-key, 28
Cup-headed bolt, 17
Decimal equivalents, 6
Dimension lines, 5
Dimensions, 5
-- of box couplings, 26
-- cast-iron flange couplings, 29
-- keys, 24
-- stuffing-boxes and glands, 67
-- Whitworth screws, 15
Distance lines, 5
Dividers, 1
Draught of cotter, 49
Drawing board, 1
-- instruments, 1
-- paper, 2
-- pen, 1
-- pins, 2
Eccentrics, 47
Exhaust port, 71
Eye-bolt, 18
Fairbairn's coupling, 26
Fast and loose pulleys, 37
Feather key, 23
Flange couplings, 27
Flap valves, 68
Flat key, 22
Forge irons, 77
Forging, 79
Form of wheel teeth, 40
Forms of nuts, 16
-- rivet heads, 7
-- screw threads, 15
Foundry irons, 77
Gasket, 58
Gas threads, 15
Gib, 49
-- head, 23
Girder stay, 81
Gland, 64
Grey cast iron, 77
Gun-metal, 80
Gusset stay, 12
Half-lap coupling, 26
Hangers, 34
Hardening of steel, 80
Headstock lathe, 96
Hexagonal nut, 16
Hollow key, 22
Hook bolt, 18
Hornbeam, 81
India-rubber disc valves, 58, 68
Inking drawings, 2
Inside lap of valve, 72
Joggles, 33
Joint, knuckle, 84
Journals, 24
-- length of, 32
Junk ring, 61
Keys, 22
-- proportions of, 23
Kinghorn's metallic valve, 70
Knuckle joint, 84
-- screw thread, 15
Lancaster's piston packing, 61
Lap joints, 8, 9, 10, 12
-- of slide valve, 72
Lathe headstock, 96
Lead of valve, 74
Lever, bell crank, 86
-- ratchet, 96
Lignum-vitæ, 81
Locking arrangements for nuts, 21, 62
Lock nuts, 19
Locomotive connecting rod, 51
-- cranked axle, 45
-- cross-head, 56
Locomotive eccentric, 47
-- piston, 60
Lubricator, needle, 32
Malleable cast iron, 78
-- iron, 78
Marine connecting rod, 51
-- coupling, 28
-- crank-shaft, 46
-- piston, 61
Merchant bars, 78
Mortise wheels, 41
Mottled cast iron, 77
Muff couplings, 25
Muntz metal, 80
Needle lubricator, 32
Nuts, forms of, 16
-- lock, 19
-- proportions of, 18
Oil-box, 54, 65
Outside lap of slide valve, 72
Overhung crank, 43
-- cranks, proportions of, 45
Packing, 63
Pan head, 7
Pedestal, shaft, 30
Pencils, drawing, 1
Phosphor bronze, 80
Pillar bracket, 34
Pillow block, 30, 32
Pin, cross-head, 51, 54
-- split, 21
Piston rod, 57
Pistons, 57
Pitch circle, 40
-- of wheel teeth, 40
-- surfaces of wheels, 39, 43
Pivots, 24
Plummer block, 30
Plunger, 57
Printing, 4
Proportions of bolt-heads, 18
-- keys, 23
Proportions of lap joints, 9, 10
-- marine engine pistons, 62
-- nuts, 18
-- overhung cranks, 45
-- pillow blocks, 32
-- rivet heads, 7
-- wheel teeth, 40
Puddled bars, 78
Puddling process, 78
Pulley, eccentric, 47
Pulleys, 36
Pump bucket, 58
Ramsbottom's packing, 60
Ratchet lever, 96
Riveted joints, 8
Rivet heads, forms of, 7, 8
-- -- proportions of, 7
Riveting, 7
Rivets, 6
Rope pulley, 39
Round key, 23
Saddle key, 22
Scales, 5
Screw coupling, 96
Screwed gland and stuffing-box, 65
Screw threads, 14, 15
Screws, representation of, 16
Sellers =V= screw thread, 14
Set screw, 21, 49
-- squares, 1
Shaft couplings, 25
-- hanger, 34
Shafting, 24
Shear steel, 79
Sheave, eccentric, 47
Shingling, 78
Shrinking, process of, 44
Siemens-Martin steel, 79
Slide blocks, 56
-- valves, 68, 71
Sliding key, 23
Snap head, 7
Snug, 17
Spiegeleisen, 80
Spring bows, 1
Spur wheel, 41
Square nut, 16
-- screw thread, 14
Stay, back, for lathe, 86
-- bar, 83
-- girder, 81
-- gusset, 12
Steam ports, 71
-- whistle, 96
Steel, 79
Steps, 30
Strap, 49
-- eccentric, 47
-- end of connecting rod, 49
Stud bolts, 18
Studs, 18
Stuffing-boxes, 63
Sunk key, 22
Taper bolt, 18, 27
-- pin, 23
Tee-headed bolt, 18
Tee-iron cover strap, 81
Tee square, 1
Teeth of wheels, form and proportions of, 40
Teeth, pitch of, 40
Tempering of steel, 80
Throw of crank, 44
-- eccentric, 47
Toothed wheels, 39
Valve Kinghorn's metallic, 70
-- slide, 68, 71
Valves, 68
-- conical disc, 70
-- india-rubber, 58, 68
Velocity ratio in belt gearing, 36
Wall boxes, 34
Washers, 19
Welding, 79
Whistle, steam, 96
White cast iron, 77
Whitworth screws, dimensions of, 15
-- =V= screw thread, 14
Wood, 81
Working drawings, 4
Wrought iron, 78
Yellow pine, 81
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